A monarchy is a form of government in which authority is actually embodied in a single individual (the monarch). When the monarchs has no or few legal restraints in state and political matters, it is called an absolute monarchy and is a form of autocracy. Cases in which the monarch's discretion is formally limited (most common today) are called constitutional monarchies. Inhereditary monarchies, the office is passed through inheritance within a family group, whereas elective monarchies are selected by some system of voting.
Historically these systems are most commonly combined, either formally or informally, in some manner. (For instance, in some elected monarchies only those of certain pedigrees are considered eligible, whereas many hereditary monarchies have legal requirements regarding the religion, age, gender, mental capacity, and other factors that act both as de facto elections and to create situations of rival claimants whose legitimacy is subject to effective election.)
Monarchy was the most common form of government into the 19th century, but it is no longer prevalent, at least at the national level. Currently, 44 sovereign nations in the world have monarchs acting as heads of state, 16 of which are Commonwealth realms that recognize Queen Elizabeth II as their head of state. All European monarchies are constitutional ones, with the exception of the Vatican City, but sovereigns in the smaller states exercise greater political influence than in the larger.
The monarchs of Cambodia, Japan, Jordan, Malaysia and Morocco "reign, but do not rule" although there is considerable variation in the amount of authority they wield. Although they reign under constitutions, the monarchs of Brunei, Oman, Qatar, Saudi Arabia and Swaziland appear to continue to exercise more political influence than any other single source of authority in their nations, either by constitutional mandate or by tradition.