 The veto was an essential component of the Roman conception of power being wielded not only to manage state affairs but to moderate and restrict the power of the state's high officials and institutions.In Westminster systems and most constitutional monarchies, the power to veto legislation by withholding the Royal Assent is a rarely used reserve power of the monarch.In practice, the Crown follows the convention of exercising its prerogative on the advice of its chief advisor, the prime minister.