What is vesting, and what is a cliff?
Vesting means equity is earned over time rather than owned outright from day one. A cliff is an initial period during which nothing vests at all.
The standard arrangement. Four-year vesting with a one-year cliff:
Nothing vests for the first twelve months.
At twelve months, 25% vests at once.
The remainder vests monthly or quarterly over the following three years.
What the cliff is for. It protects the company from someone leaving after a few weeks with a permanent stake. Before the cliff, a departure means they leave with nothing — which is harsh and is exactly the intended effect.
Why founders should vest too. This is the part first-time founders resist and later regret. Consider two founders splitting equity equally, one of whom leaves after four months. Without vesting, the leaver keeps half the company permanently, while the remaining founder does all the work and can neither buy them out nor raise investment against a cap table with a large passive holder.
This situation is common, unfixable after the fact, and has killed companies. Investors will insist on founder vesting if it is not already in place, frequently restarting the clock — so agreeing it early is in the founders' own interest.
The mechanisms. For unissued shares, options simply vest. For shares already issued, the equivalent is usually reverse vesting — the shares are held but the company can buy back the unvested portion at nominal value if the holder leaves.
Good leaver / bad leaver provisions determine what happens on departure. A good leaver typically keeps vested equity; a bad leaver — dismissed for cause, or in breach — may lose more, sometimes including vested shares. The definitions are negotiable and matter a great deal; overly broad bad leaver clauses are worth resisting.
Acceleration clauses vest equity early on a sale. Single trigger vests on acquisition; double trigger requires acquisition and the person being terminated, which is the more usual and more defensible form.
General information, not legal advice.