The appointment of a Prime Minister by the monarch is formal, based on advice given to them. However, though the advice is "technically" informal, the monarch would create a constitutional crisis if they did not comply. Therefore, in practice, what is decided in the House of Commons is the decisive event. The voters elect Members of Parliament (MPs), and the MPs choose the Prime Minister. The system grew up in stages as the Commons grew in power, and the monarch's authority dwindled. 