Vesting
The schedule over which equity is genuinely earned, typically four years with a one-year cliff.
Vesting means shares or options are earned over time rather than owned outright at grant. The standard is four years with a one-year cliff: nothing vests for twelve months, then 25% vests at once, then monthly thereafter.
Founder vesting protects the remaining founders as much as the investors. Without it, a co-founder who leaves in month three keeps their full stake, and the people who stay carry the company for someone who did not.
Acceleration clauses modify this on acquisition. Single trigger vests on the deal itself; double trigger requires both the deal and the employee being terminated. Double trigger is standard and is what acquirers expect to see.