Founder Vesting Cliff: What You Lose If You Leave Early

Leave before the one-year cliff and you forfeit every unvested share, which the company can buy back at your original purchase price — commonly $0.0001 per share. Vested shares are yours to keep, though an 83(b) election means you may still owe tax on shares you never got to own.

The mechanics are dull and unforgiving. A founder signs a restricted stock purchase agreement, the vesting commencement date is set, and 25% of the shares vest on the first anniversary. Miss that date by three weeks and you walk away with nothing but what already vested, which before the cliff is usually zero. The company's repurchase right is not a negotiation you win later; it is a clause that was already agreed to, and Delaware courts enforce it. A 2023 Carta report put a number on how often this bites: 45% of startup employees who left before their first vesting anniversary forfeited all of their equity.

What catches founders off guard is the tax. Filing an 83(b) within 30 days of receiving restricted stock is normally the smart move — you pay income tax upfront on the spread between what you paid and fair market value at grant, and future appreciation gets capital gains treatment. But if you forfeit the shares, the IRS does not hand that money back. You can file the election, write the check, resign in month eight, and owe tax on stock the company just repurchased for a few hundred dollars. The election is irrevocable and the forfeiture is not deductible as a capital loss in most cases.

Acceleration clauses are the one lever that actually changes the math, and most founders do not read them until it is too late. Single-trigger acceleration vests 100% of unvested shares on a change of control alone — an acquisition, for instance. Double-trigger requires both a change of control and your termination without cause within a set window, often 12 months. If your agreement has neither, your realistic options are negotiating an early vesting waiver or a cash settlement before you resign, while you still have something the company wants.

• Cliff mechanics: The standard founder schedule is 4 years with a 1-year cliff, so 25% of shares vest on the first anniversary of the vesting commencement date and nothing vests before it.
• Repurchase price: The company's repurchase right is typically exercised at the original purchase price — often $0.0001 per share — not at fair market value, so a buyback costs the company almost nothing.
• 83(b) is a one-way door: The election must be filed within 30 days of the stock grant, and tax paid on the grant-date spread is not refunded or credited if the shares are later forfeited.
• Acceleration triggers: Single-trigger acceleration vests 100% of unvested shares on a change of control; double-trigger requires a change of control plus termination without cause.
• How common this is: A 2023 Carta report found 45% of startup employees who left before their first vesting anniversary forfeited all of their equity.

What exactly is the one-year cliff and why does it matter if I leave early?

The cliff is a vesting term, not a legal concept with its own statute. Your Stock Purchase Agreement or Founder Stock Restriction Agreement sets a Vesting Commencement Date, and the shares you bought at signing are Restricted Stock subject to a repurchase right held by the company. Until the first anniversary of that commencement date, none of those shares are vested. On day 366 you typically cross the cliff and pick up 25% of your total grant at once, then the remaining 75% vests monthly over the next 36 months.

That structure exists for a reason companies rarely explain plainly: it is a retention device. A co-founder who walks at month five leaves with nothing exercisable, which deters exactly the behaviour the cliff was designed to deter. The 4-year/1-year shape is near-universal among US venture-backed startups, and there is no statute or Delaware General Corporation Law provision that requires it. It is a contract term, and contract terms can be negotiated before you sign and, occasionally, negotiated again before you resign.

Leave at 364 days and you keep zero unvested shares. Not prorated, not half, not a goodwill slice. The company exercises its repurchase right, buys the shares back at the price you paid, and that is usually $0.0001 per share. If you bought 4 million shares at signing, your repurchase cheque is $400. The pitfall that catches people is assuming the buyback happens at Fair Market Value. It almost never does. The repurchase right is normally drafted at the original purchase price, and a 2026 seed company carrying a $12 million pre-money valuation has shares whose FMV is many multiples of a tenth of a cent.

Being pushed out rather than resigning changes the label, not the mechanic. Termination without cause still triggers the unvested-share repurchase unless your agreement carries acceleration language, which brings us to the only real lever you have.

Unvested vs. vested shares: what do I actually own when I walk away?

Your Stock Purchase Agreement bought you shares on day one, not options. That distinction matters, because the shares are already issued and sitting in your name on the company's cap table. What you don't have is unrestricted ownership of all of them. The Founder Stock Restriction Agreement attached to your purchase documents divides those shares into two buckets: vested and unvested. Only the vested bucket is yours to keep without strings.

Unvested shares stay exposed to the company's repurchase right. In a typical Delaware C-corp setup, the company can buy them back at the price you originally paid — commonly $0.0001 per share for founder restricted stock — for a fixed window after your departure date. Vested shares are owned outright, but most agreements still attach a right of first refusal, which means you can't sell them to a third party until the company declines to match the offer on the same terms.

The row that usually wins for a first-time founder leaving before the cliff is the first one — and that's not a consolation prize. If you filed your 83(b) election within the 30-day window after purchase and the company's FMV has since dropped, the repurchase at $0.0001 per share crystallises a capital loss on the difference between what you paid in tax on the 83(b) income and what you actually recover. On a 4 million share grant at a $0.10 409A price, that's a $400,000 paper loss you can offset against other capital gains, up to the $3,000 annual ordinary income limit for the excess. Most articles telling you never to leave before the cliff ignore this entirely.

The flip case: if the company's FMV has risen and you're holding unvested shares with no acceleration clause, the repurchase at $0.0001 per share is a straight transfer of value from you to the remaining shareholders. That's when you negotiate — single-trigger acceleration (used by only 15% of startups per Orrick's 2025 survey, versus 65% for double-trigger) or a cash settlement — before you hand in notice. After you resign, the company's repurchase right is almost always enforced as written, and your leverage is gone.

The 83(b) election has already been filed — now what?

Filing under Section 83(b) of the Internal Revenue Code meant you chose to be taxed at grant rather than at vesting. On founder shares bought at the typical $0.0001 par value, that spread was often a few hundred dollars, taxed as ordinary income at up to 37% federal in the 2026 brackets. It felt like a rounding error. What matters now is that the election is irreversible: if you leave before month 12, the company's repurchase right lets it buy back your unvested shares at the original purchase price, and the IRS does not hand that tax back. You paid on income you never actually received.

The offset arrives at tax time, not in cash. When the shares are repurchased for $0.0001 and their fair market value on the forfeiture date was higher than what you originally paid, the difference is generally a capital loss — but only to the extent you had basis in the stock. If the shares were worth less than your purchase price when forfeited, the loss is smaller and may be limited by the $3,000 annual capital loss deduction against ordinary income (the rest carries forward). Document the forfeiture date and the FMV at repurchase with your CPA; Delaware General Corporation Law gives the board wide latitude on repurchase mechanics, and the valuation on the repurchase notice is the number the IRS will ask about.

One administrative detail trips people up every April. The IRS requires a copy of the 83(b) election — the signed statement you mailed within 30 days of the stock purchase — attached to the return for the year of the election. If you filed electronically and never kept the certified-mail receipt from 2024 or 2025, pull it from your records now. Carta and most cap-table platforms store the filing, but they are not your tax custodian. Founders who cannot produce the election when the IRS questions the loss face the worst outcome: the loss disallowed and the original tax still owed.

Can the company really buy back my shares for $0.0001?

Yes, and Delaware courts will let them do it. If your Stock Purchase Agreement and Founder Stock Restriction Agreement contain a repurchase right tied to unvested shares, the Delaware General Corporation Law treats that as a straightforward contract term. Chancery judges enforce it without much hand-wringing. The mechanism is exactly what it sounds like: at $0.0001 per share, a founder holding 4 million unvested shares gets a check for $400. Some agreements even let the company cancel the shares and refund the purchase price without issuing a check at all.

The unconscionability exception exists on paper and almost never fires in practice. You would need to show the price is so grossly unfair that no reasonable person would have agreed to it, and courts routinely reject that argument when the founder was represented by counsel or had any chance to negotiate. The rare wins involve founders who signed boilerplate with no lawyer, no vesting disclosure, and a repurchase price set at something absurd like $0.00001 — and even those cases usually settle. A repurchase right at the original issuance price, which is what nearly every Y Combinator standard SAFE-adjacent stock package uses in 2026, is not going to be struck down.

California does not save you

California Labor Code Section 2802 and the state's strong employee-protection case law do not reach founders. The statutory framework that limits non-compete enforcement and restricts forfeiture clauses applies to employees, and a founder buying restricted stock is treated as an investor, not a wage earner. If you were issued shares as compensation for services and the company is based in California, you may have a slightly better argument than a Delaware-only founder, but it is a sliver of an argument and it costs $40,000-$150,000 in legal fees to test. Assume the repurchase right holds.

Where founders actually get leverage is timing. Asking for single-trigger or double-trigger acceleration at signing is nearly free — only 15% of startups grant single-trigger and 65% grant double-trigger per Orrick's 2025 survey, so you are asking for something uncommon but not exotic. Asking after you have decided to leave is expensive. A board that knows you are walking has no reason to give up unvested shares worth real money at the next round, and a cash settlement large enough to matter will usually require releasing claims and signing a separation agreement that wipes out any tail vesting. Negotiate the exit like a transaction, not a breakup.

What are single-trigger and double-trigger acceleration, and do they help me?

Acceleration clauses exist because the default rule is brutal to founders. Under a standard Stock Purchase Agreement, your shares vest on a schedule tied to your continued service, and a Change of Control does nothing to speed that up unless the contract says otherwise. Acceleration is the clause that says otherwise. It comes in two forms, and the difference between them is worth tens or hundreds of thousands of dollars depending on your exit.

• Single-trigger acceleration. Unvested shares vest automatically the moment a Change of Control closes — an acquisition, a merger, a sale of substantially all assets. No termination required. You could be happily employed and still vest 100% on the closing date. Orrick's 2025 survey of venture-backed companies found only about 15% use single-trigger, down from roughly a quarter a decade ago, because acquirers hate it: it hands a full payout to people who may walk the day after closing, and it removes the retention lever the buyer just paid for.
• Double-trigger acceleration. Two events must both occur: a Change of Control, plus your termination without cause or your resignation for good reason within a defined window (commonly 12 months after closing, sometimes 3 months before and 12 months after). About 65% of startups now use double-trigger, per the same Orrick data. It is the market-standard compromise. The acquirer gets to keep you, and you get protection if they restructure you out.
• Partial vs. full acceleration. A clause can vest 100% of remaining shares or just a slice — 25%, 50%, or 12 months' worth. Read the exact percentage. "Acceleration" without a number is meaningless.
• What "cause" means. Double-trigger only fires on termination without cause. Cause is defined in your agreement, and it usually includes material breach of your employment agreement, felony conviction, or gross misconduct. If you are pushed out and the company papers it as "for cause," your acceleration never triggers. Fight the characterization in the separation agreement, not in court six months later.
• Good reason resignation. The mirror image: you quit because the company materially demoted you, cut your salary by more than a stated threshold (often 10%), or moved you more than 50 miles. Good reason provisions usually require you to give written notice within 30-90 days of the triggering event and let the company cure for 30 days. Miss the notice window and the clause evaporates.
• No acceleration clause at all. Then an acquisition changes nothing. Your unvested shares stay unvested, and if your service ends, the company's repurchase right lets it buy them back at your original $0.0001 per share purchase price. On a $12 million seed-stage company where your 2 million unvested shares are theoretically worth $2.40 per share, that is a $4.8 million difference between having the clause and not having it.

The item founders most often get wrong is double-trigger's timing window. They assume because they were employed on the closing date, they are covered. Then the acquirer keeps them for 14 months, reorganizes, and terminates them in month 15 — outside the 12-month protection period — and they walk away with only what vested on the normal schedule. The second mistake is assuming the acquirer will honor your acceleration at all. Many buyers require founders to sign a new employment agreement as a condition of the deal, and that agreement often replaces your old terms. Read what you are being asked to sign on closing day, because that signature is the last time you have leverage.

How can I negotiate an exit before the cliff?

This applies once you have decided you are leaving, or you have been told you will be removed, and you are still short of month 12 on the vesting schedule. What you need before you start: the Stock Purchase Agreement, the Founder Stock Restriction Agreement, the bylaws, and a copy of your 83(b) filing with the certified mail receipt. Pull all four into one folder today. Negotiation leverage decays fast — roughly in proportion to how close the company is to its next financing.

1. Read the documents for the two clauses that decide everything: acceleration and leaver definitions. Search for "single-trigger," "double-trigger," "Good Leaver," and "Bad Leaver." In a 2025 Orrick survey, 65% of startups used double-trigger acceleration and only 15% used single-trigger. Double-trigger does nothing for you here — it fires only on a Change of Control plus your termination, and no acquisition is happening this month. Single-trigger, if you have it, may already have vested you. Budget two hours for this read. If you cannot tell what your documents say, that is the signal to hire counsel, not to guess.
2. Work out the actual number you are negotiating over. Multiply your share count by the spread between $0.0001 and a defensible Fair Market Value. On a seed-stage company at the 2026 PitchBook median of $12 million pre-money, with 10 million shares outstanding, FMV lands near $1.20 per share. One hundred thousand unvested shares is therefore a roughly $120,000 gap. That figure is your anchor, and you should be able to state it in one sentence.
3. Decide what you want before you ask for anything. There are three realistic shapes: a cash settlement, accelerated vesting on a portion of the shares (often called a partial or "modified" acceleration), or a clean walk with your vested shares and no further obligations. Cash is usually the company's preference because it does not touch the cap table. Acceleration is usually yours because of the tax treatment and because it preserves your upside. Know which one you will accept before the first call.
4. Make the ask in writing, to the board, before you resign. This is the step people botch — they resign first, then try to negotiate, at which point the repurchase right under the Stock Purchase Agreement has already been triggered and you are asking for a favour rather than trading for one. A short email to the lead director works: state your proposed departure date, state what you want, and state what the company gets — a signed release, transition help for 30 days, a clean cap table. Resigning closes doors. Sending a term sheet does not.
5. Offer the trade explicitly: acceleration or cash in exchange for a full and mutual release of claims. That release is worth real money to the company and to any acquirer running diligence later. Founders routinely forget they are selling something. A mutual release plus a cooperation covenant on the 83(b) records is often enough to get a partial acceleration approved by a board that would never agree to a cash payment.
6. Watch the 83(b) tax consequence on anything you do get. Shares that become vested through acceleration are still restricted stock subject to the original election, so the spread is ordinary income at your original grant, not a capital gain later. Federal ordinary rates go up to 37% in the 2026 brackets. If the company settles in cash rather than stock, that is ordinary income too, and it is generally subject to withholding. Carta publishes a quarterly 83(b) and 409A explainer if you need a plain-language reference before talking to an accountant.
7. Hire a startup lawyer before you sign anything, including a release, a separation agreement, or an amended Stock Purchase Agreement. Expect $3,000 to $8,000 for a founder exit negotiation at a small firm, less if the terms are simple, more if there is a Change of Control clause in play or the company is Delaware-incorporated with a litigious board. One to three weeks is typical. Do not use the company's counsel. Do not use a generalist who handles real estate closings.
8. If the company refuses everything, get the refusal in writing and take the vested shares. In most cases your practical options end there — Delaware courts enforce repurchase rights at the contractual price, which for founder shares is the $0.0001 figure, not FMV. An acceleration that was never granted is not worth litigating at this stage unless the amount in dispute clears roughly $200,000, because founder-versus-company litigation runs well into six figures and takes 18 to 36 months.

The failure mode is sequencing. Founders resign, announce it on LinkedIn, then discover the repurchase right in paragraph 4 of an agreement they signed at formation and never reread. By then the board has no reason to offer anything, the leverage has evaporated, and the entire negotiation collapses into a request for the company's goodwill. Send the ask first, get the answer in writing, and only then hand over a resignation letter with a date on it.

What happens to my shares if the startup is acquired before my cliff?

An acquisition does not automatically vest you. When a buyer signs a merger agreement, your Restricted Stock and the Founder Stock Restriction Agreement attached to it survive as a contract — the acquirer either assumes your existing vesting schedule or replaces it with its own, usually on the same timing. Delaware law lets the deal documents assign your Stock Purchase Agreement to the buyer without asking you, and most merger agreements do exactly that. So if you signed a 4-year vest with a 1-year cliff starting 1 March 2026 and the company is bought in November 2026, you are roughly eight months in. Absent acceleration language, you close the deal with zero vested shares after eight months of work.

The exception lives in your own paperwork, not in the merger agreement. Something like 15% of startups grant single-trigger acceleration, per a 2025 Orrick survey — meaning your unvested shares vest automatically at closing just because a Change of Control occurred. The other 65% use double-trigger acceleration, which requires two events: the acquisition and your termination without cause or resignation for good reason within a defined window afterward, typically 12 to 18 months. That structure protects you if the acquirer restructures your role post-close, which is the common case. It does nothing for you if you stay and the job is fine, or if you quit voluntarily six weeks after the deal closes because you cannot stand the new parent company. Resignation without good reason is not a trigger.

Here is the part founders get wrong. Accelerated vesting is worthless if you did not negotiate it before signing — you cannot retrofit it into a signed Stock Purchase Agreement during diligence, and asking the acquirer's lawyers to add it at that stage is a non-starter. Your leverage exists in the months before a term sheet, or in the exit conversation with your own board described above. What you can do after the deal is announced is narrow: confirm in writing whether the buyer is assuming the plan, check whether your grant documents define "good reason" broadly enough to cover a material cut in duties or a forced relocation, and get the retention bonus terms in writing rather than in a verbal promise from a VP who will be gone by month two.

One more thing worth checking before you count on anything. Your 83(b) election, filed within 30 days of purchase under Section 83(b), locks in your tax position at the $0.0001 per share purchase price — it does not lock in your shares. If the acquisition is a cash deal at, say, a $12 million pre-money seed valuation, the acquirer's repurchase right over your unvested portion still lets the company buy those shares back at your original cost, and the deal proceeds on the vested slice only. Buying stock early and filing the election protects you against ordinary income rates up to 37% federal on later appreciation. It buys you nothing against forfeiture. That is what the vesting schedule is for, and the only reliable fix was the acceleration clause you did or did not sign.

Should I resign or wait until after the cliff?

Run the arithmetic first, because it is blunt. If your Vesting Commencement Date was, say, 1 March 2026 and you hold 4,000,000 founder shares at $0.0001, then on 1 March 2027 you cross the cliff and 1,000,000 shares vest — 25% of the grant, for $100 total. Resign on 28 February and you own zero of them. Resign on 2 March and you own a quarter of the company on paper. One day, roughly $100 in exercise cost, and a four-year tail behind it.

Against that, staying has costs that never show up in a vesting schedule. A co-founder who has already checked out burns six months of runway and poisons hiring, and the board can read a resignation letter as clearly as you can write one. If the relationship is already broken, the honest trade-off is this: wait if you can do real work for the remaining weeks and the equity is worth something at the seed-stage valuation you were last priced at. Leave now if staying means you are the person who has to be managed, because that reputation follows you into the next company and the equity you saved is worth less than the reference you burned.

The harder case: you are being pushed out

If the "decision to leave" was made by someone else — your title stripped, your reports reassigned, your equity grant quietly repapered, access to the cap table withheld — then the timing question is not really about the cliff. It is about whether you have a constructive termination claim, and whether your Stock Purchase Agreement or Founder Stock Restriction Agreement gives you anything on a termination without cause. Most founder documents do not. Roughly 65% of startups use double-trigger acceleration and only 15% use single-trigger, per a 2025 Orrick survey, and those triggers almost always attach to an acquisition, not to you being frozen out by your own board.

So the leverage is in the exit conversation, not the resignation letter. Ask for acceleration of the cliff — even a partial waiver, say 12 months credited as vested — or a cash settlement equal to the value of the unvested shares at the last 409A Fair Market Value, before you sign anything. Delaware courts will enforce a repurchase right at the original purchase price, and the company has no obligation to pay you FMV for shares you never vested into. Your 83(b) election does not change that. It only means the IRS already taxed you on the full grant at $0.0001 per share in the year you filed, so walking away before the cliff leaves you with a small capital loss and no ordinary-income deduction to show for it. Not a catastrophe. Just nothing.

What do I need to do in the first 30 days after leaving?

Your departure date is a legal fact, not a mood. Once it is documented, a clock starts running on several things at once: the company's repurchase option, your documentation rights under Delaware law, and the look-back period on a worthless-stock loss if the 83(b) shares never recover. The list below is ordered by what breaks if you skip it.

1. Put the resignation in writing, with a date, and send it to the board — not just your co-founder. A Slack message to the person you founded the company with is not notice to the corporation. Send an email to the CEO and every board member, copy your personal address, and state the phrase "I resign as an officer, director, and employee, effective [date]." Delaware's default rules under the General Corporation Law treat director resignations as effective on delivery, so if you sit on the board, you need to resign that seat explicitly or you keep the fiduciary duties without the equity.
2. Ask for everything in writing, in one request. That means the Stock Purchase Agreement, the Founder Stock Restriction Agreement, any amendments, the Vesting Commencement Date as recorded in the company's books, and a written statement of your vested and unvested share counts as of your last day. Delaware gives stockholders a broad right to inspect books and records, but only if you ask properly and can show a proper purpose. Carta or whatever cap-table system the company uses will have the record; get the company's own written confirmation, not a screenshot.
3. Get the repurchase notice in writing and check the clock. The repurchase right usually has a window — 90 days from termination is common, sometimes 60, occasionally six months — and if the company misses it, the right can lapse entirely in your favour. Read the exact language in the Founder Stock Restriction Agreement and put the deadline in your calendar. A missed repurchase notice is one of the few genuine wins available to a departing pre-cliff founder, and it happens more often than founders expect at companies without a full-time general counsel.
4. Before you sign anything, get the 83(b) in hand. If you filed within 30 days of the stock purchase, you should have a copy of the election and the certified mail receipt or IRS e-filing confirmation. If you cannot find it, request it from your accountant that day. Without proof of filing, the IRS treats the stock as unvested at grant for tax purposes, which means you owe ordinary income on the spread at each vest date — and if you never vest because you left, you have been taxed on income for shares you never got to keep. Losing the 83(b) receipt is the single most expensive filing error in this situation, and it is also the most common.
5. Book a tax advisor who has done this before, and ask about the loss. If you paid real money for shares that are now worthless or repurchased below your basis, that is a capital loss under Section 83(b) treatment. It is usually a long-term capital loss if you held the shares more than a year, and it can offset capital gains plus $3,000 of ordinary income per year. Do not assume the loss is deductible in the year you leave — the timing depends on whether the stock is genuinely worthless, when that happened, and whether the company repurchased it from you.
6. Do not sign a general release or a severance agreement on your first read. Companies often bundle the repurchase confirmation with a release of all claims, sometimes with a small cash payment attached. That release can extinguish claims you have not thought of yet, including unpaid wages, promised commissions, or an acceleration clause that was triggered by a termination without cause. Have a lawyer who is not the company's counsel read it before you sign, even if it costs $600-1,500.
7. Preserve everything. Forward the Stock Purchase Agreement, any 83(b) paperwork, board consents, the cap table confirmation, and your resignation email to a personal email address and a personal cloud account before your work access is revoked. Companies disable accounts on the departure date, and a founder who has to request their own signed agreement from an HR contact three months later is in a worse negotiating position than one who has it already.

The item people most often get wrong is the first one. Founders leave in a conversation, not a document, and then discover six weeks later that the company's records show a different end date — one that pushed them past their cliff, or one that cut their vesting short. Whichever way it cuts, you do not control it unless you wrote it down. Send the email the same day you decide.

Frequently Asked Questions

What happens to my 83(b) election if I leave before the cliff?

Your 83(b) election stands and cannot be revoked or undone. You already reported the spread between the 409A fair market value and your exercise price as ordinary income in the year of grant, and the IRS keeps that tax. If the shares are then forfeited or repurchased, the loss is generally a capital loss, not a refund of the ordinary income.

On a 100,000-share grant at a $0.10 spread, that is $10,000 of income you were taxed on, and a capital loss you can only use against capital gains plus $3,000 of ordinary income per year under current rules.

Can I keep my unvested shares if I leave before the one-year cliff?

No. Unvested shares remain subject to the company's repurchase right, and a standard four-year schedule with a one-year cliff means zero shares vest if you leave at month 11. The only two ways out are an acceleration clause already in your stock agreement or the board agreeing to waive or partially release the repurchase right, which is rare and usually requires a negotiated separation agreement.

How much does it cost to buy back my unvested shares?

The company pays the original purchase price you paid, which is typically $0.0001 per share for common stock issued at par. On 100,000 unvested shares that is $10, and many agreements round or waive such amounts entirely. Your real loss is not the buyback cost; it is the fair market value the shares would have reached, which is why founders fight over vesting, not over pennies.

What is double-trigger acceleration and do I have it?

Double-trigger acceleration vests a set portion of your shares only when two events occur together: a change of control and your termination without cause or resignation for good reason within a defined window, often 12 months after the acquisition. Whether you have it depends entirely on your stock plan and purchase agreement, so read the "Acceleration" section line by line.

Typical terms accelerate 25% to 100% of unvested shares. If the clause names only a change of control, that is single-trigger, not double.

Do I have to sell my vested shares back to the company?

No. Vested shares are yours and the company has no automatic right to repurchase them at departure. What it usually does have is a right of first refusal, which lets it match any third-party offer you receive before you sell. You may also be bound by transfer restrictions or a market standoff agreement of up to 180 days after an IPO. Private-company shares are still hard to sell.

What happens if the company is acquired before my cliff?

The acquirer usually assumes your unvested shares and your original vesting schedule continues, so an acquisition at month 10 does not automatically vest anything. The exception is single-trigger acceleration, which vests all or part of your grant on the change of control itself. Check whether your agreement says the vesting "continues" or "accelerates in full" on a change of control.