An acceleration clause decides who keeps unvested equity when the company is sold.

Two different clauses share the name

In lending, an acceleration clause lets a lender declare the entire outstanding balance immediately due on a default — the loan is "accelerated" to maturity. In equity, the term is unrelated: it accelerates a vesting schedule so shares vest earlier than the calendar would allow. Everything below is the equity meaning.

Acceleration only matters because vesting exists (/equity/vesting). A four-year schedule assumes four years of employment at one company. An acquisition breaks that assumption, sometimes eighteen months in. Acceleration terms are the contractual answer to the question of who bears that break.

Single trigger

Single trigger accelerates vesting on the change of control alone. The deal closes, the unvested portion — all of it, or a stated fraction — vests immediately, and the holder is treated as if they had served the remaining term. Nothing else has to happen. They can stay, leave the next morning, or have already been told their role is redundant; the acceleration has fired.

That is genuinely valuable to the holder and genuinely expensive to everyone else, which is why it is negotiated hard and rarely granted broadly. Where it appears, it is often partial: a stated percentage of the unvested balance, or twelve months of additional vesting credit rather than the whole remainder.

Double trigger

Double trigger requires two events. First, a change of control as defined in the document. Second, a qualifying termination within a defined window after closing — commonly twelve months. Both are needed. A deal with no termination accelerates nothing; a termination with no deal accelerates nothing.

A qualifying termination is normally an involuntary termination without cause, or a resignation for good reason. The first half is straightforward. The second half is where the clause is won or lost.

Two supporting mechanics are worth reading alongside it. Most definitions require notice and a cure period: the holder must flag the condition in writing within a set number of days and give the company a chance to fix it before resigning. Miss the notice window and good reason is waived. And the post-closing protection window has an end date — a termination in month fourteen under a twelve-month window is just a termination.

How much accelerates: full, partial, or vesting credit

Single and double describe what fires the clause, not how much it releases. Three shapes are common and they are not interchangeable. Full acceleration vests the entire unvested balance. Partial acceleration vests a stated fraction of it — half the remainder, say — leaving the rest on the original schedule. Vesting credit adds a fixed period of service, typically twelve or twenty-four months, and vests only what that additional time would have released.

The difference between a fraction and a credit is largest at the extremes of a schedule. Someone eleven months into a four-year grant with a twelve-month cliff has vested nothing; twelve months of vesting credit carries them past the cliff and releases a quarter of the grant, while half of the unvested balance releases far more. Someone forty months in has little unvested left, so twelve months of credit and full acceleration converge on nearly the same number. Read the clause against the actual position rather than in the abstract.

Watch for discretionary language as well. Most plans give the board or the plan administrator power to accelerate awards in connection with a transaction. That is a permission, not an entitlement: it can be exercised for some holders and not others, and it can be declined entirely. A grant that says the board "may" accelerate confers nothing you can rely on. A grant that says vesting "shall" accelerate on defined events is the one worth having.

What "change of control" actually means in the document

Change of control is a defined term, not a fact about the world. Two companies can experience the same transaction and reach opposite conclusions because their plan documents define it differently. Read the definition before reading the acceleration clause; the trigger is upstream of everything.

The recurring distinctions: a stock sale or merger, where ownership of the entity itself changes hands, is almost always covered. An asset sale, where the buyer takes the business but the legal entity survives holding cash, is sometimes excluded — and that is exactly the structure in which employees are most likely to lose their jobs. A financing round that transfers majority ownership to new investors can meet a literal ownership-percentage definition, so well-drafted definitions carve out bona fide equity financings. A reincorporation or holding-company reorganisation moves 100% of the stock to a new parent and is normally carved out too, because nothing economic happened.

Board-composition tests appear as a second limb in some documents: a change of control also occurs if a majority of the board is replaced within a defined period other than by the incumbent directors’ approval. That catches control changes that never show up as a share transfer.

What happens to unvested equity when there is no acceleration

Most grants have no acceleration term at all, and the acquisition agreement decides their fate. There are four common outcomes, and they are meaningfully different for the holder.

Why acquirers dislike single trigger

An acquirer buying a company is partly buying its people, and unvested equity is the instrument that keeps those people there. Single trigger destroys that instrument at the exact moment the acquirer starts relying on it. On closing day, an engineer with two years of unvested equity becomes an engineer with no unvested equity and a liquid position.

The acquirer’s response is predictable: fund a new retention pool of acquirer equity or cash, on top of the purchase price, to rebuild the hold it just paid to dissolve. And because acquirers price a deal on total cost rather than headline price, that new pool almost always comes out of the consideration paid to shareholders. This is the part that gets skipped in the offer-letter conversation: single trigger is not free money from the buyer. It is a transfer from the whole cap table — every common holder, every employee without acceleration, every investor — to the accelerated holders. That is why it is defensible for a small number of people whose upside was the reason the company exists, and hard to defend as a plan-wide default.

Who commonly gets what

Treatment is usually tiered, and it is worth stating this as what tends to get negotiated rather than as a measured distribution. Founders and senior executives most often have double trigger written into their grant agreements or employment agreements, and sometimes single trigger on a portion of the unvested balance. Board members and advisors are negotiated one at a time. Rank-and-file employees frequently have nothing beyond whatever the plan document says by default, which commonly leaves the outcome to the acquisition agreement.

If you are reading a grant, the practical question is not "do I have acceleration" but "which document would contain it". Acceleration can live in the plan, in the individual grant agreement, in an employment agreement, or in a separate change-of-control severance agreement. Silence in one of them does not mean silence in all of them.

The §280G golden-parachute interaction

Acceleration terms and 280G analyses turn up in the same deal folder, and the reason is arithmetic. IRC §280G applies to payments contingent on a change in control made to a disqualified individual — officers, holders of more than 1% of the stock, and certain highly compensated individuals. Accelerated vesting is counted, because the benefit is contingent on the change in control. If the total of those contingent payments reaches three times the individual’s "base amount" — their average annualised W-2 compensation over the five preceding taxable years — the excess over the base amount is an excess parachute payment.

The consequences land on both sides. The recipient owes a 20% excise tax under IRC §4999, on top of ordinary income tax. The employer loses its deduction for the excess under §280G(a). The threshold is a cliff, not a taper: crossing three times the base amount exposes everything above the base amount, so a small increase in accelerated value can produce a large tax result. A recently promoted executive is particularly exposed, because the base amount is calculated on five years of historic pay that does not reflect the current role.

Private companies have a route out that public companies do not. The shareholder-approval exemption in §280G(b)(5) allows a privately held corporation to cleanse the payments if the required disclosure is made and the requisite shareholders approve them, with the affected individual agreeing to waive the payment if approval is not obtained. That is the "280G waiver and vote" that appears in signing checklists, and it is why acceleration language gets reviewed by tax counsel long before a deal is announced. Whether any of this applies to a particular person is entirely fact-specific.

The §422(d) $100,000 ISO limit — the sleeper cost

This one is almost never discussed and it can quietly change the tax character of an entire grant. IRC §422(d) limits the aggregate fair market value of stock — measured at grant — for which incentive stock options become exercisable for the first time in any one calendar year to $100,000. The excess is not void; it is simply treated as a nonstatutory option.

A normal four-year schedule is designed to stay under that ceiling, because it spreads exercisability across four calendar years. Acceleration collapses the schedule into one. When a change of control makes four years of options exercisable for the first time in a single year, the portion above the limit falls out of ISO treatment for that year. The practical consequence is that the spread on the excess is ordinary compensation income at exercise under §83(a) and Treas. Reg. §1.83-7, rather than the ISO treatment the holder was planning around — and for an employee it is wages, subject to withholding.