The most common way to issue them is so-called "double trigger" vesting where you have both a) a service requirement (the time) and b) an event required (like I

The most common way to issue them is so-called "double trigger" vesting where you have both a) a service requirement (the time) and b) an event required (like IPO or sale). Since you have risk of forfeiture if IPO/sale does not happen, IRS does not deem vested (and thus taxable) until both triggers are satisfied.