Vesting of options is straightforward. The grantee receives an option to purchase a block of common stock, typically on commencement of employment, which vests over time. The option may be exercised at any time but only with respect to the vested portion. The entire option is lost if not exercised within a short period after the end of the employer relationship. The vesting operates simply by changing the status of the option over time from fully unexercisable to fully exercisable according to the vesting schedule.