Malaysia is a federation of 13 states and three federal territories. The country's political system is a constitutional monarchy based on the British Westminster parliamentary system. Nine of Malaysia's states, known as the Malay States, are constitutionally run by traditional Malay rulers of royal descent (sultans). The representative head of state is the sultan (king); the sultan is chosen every five years from the ranks of the rulers of the nine Sultanates (according to the principle of rotation). The executive power of Malaysia is vested in the cabinet led by the prime minister, who is the head of government.