Equity issued to founders at incorporation is usually subject to conditions rather than owned outright from the first day. Those conditions exist because founding teams change.
The problem being solved
If shares are held unconditionally, a founder who leaves after a few months retains their full stake while the remaining team continues working for years.
That outcome is unfair to those who stay and creates a persistent obstacle, because a large inactive shareholder complicates every subsequent funding round.
Vesting addresses it by making ownership accrue over time, so that the amount retained reflects the period actually contributed.
How the mechanism operates
Shares are typically issued at the outset but become fully owned in instalments across a defined period, commonly with an initial waiting period before any portion vests.
If the founder departs before the end, the unvested portion returns to the company or is repurchased at a nominal price, depending on how the arrangement was drafted.
The precise structure, its tax treatment and its enforceability differ considerably by jurisdiction, and the legal form used in one country may have no equivalent in another.
The cliff exists to filter early exits
An initial period during which nothing vests means a founder who leaves quickly retains nothing at all. This is deliberate, because the first year is when departures are most likely.
After that point, vesting usually accrues on a regular schedule, so the retained share grows steadily rather than in large steps.
The design assumes that commitment demonstrated over an extended period is what the equity is actually compensating, rather than the act of being present at the start.
Investors treat it as a precondition
Professional investors generally require founder vesting before committing capital, and where none exists they will ask for it to be introduced as part of the round.
Their concern is that the value of their investment depends on specific people continuing, and unconditional equity gives those people no structural reason to stay.
Founders sometimes find this reasonable in principle and difficult in practice, because it can mean giving up ownership they had considered settled for years.
Acceleration clauses change the outcome
Agreements often specify that vesting accelerates on a sale of the company, either automatically or where the founder is removed following the transaction.
Acquirers usually dislike full automatic acceleration, because it can leave them having bought a company whose key people are free to depart immediately.
The compromise is a partial arrangement that vests some portion on the transaction and the remainder over a further period, though the details are negotiated case by case.