A monarchy is a form of government in which the head of state and/or government is chosen based on birthright. The monarch's title varies, but the best example is a king or a queen. Monarchies are less common in the 21st century than they were in the past, and the monarchies that do exist tend to be symbolic -- the king or queen is the head of state, representing his or her country but not actually weighing in on policy.
Almost every monarchy is based on heredity. There are some rare exceptions, such as the Vatican's elected monarchy, but, by and large, monarchies are birthrights that cannot be elected or appointed. Rather, you have to be born into it. This means that a monarchy will have a royal family, to whom every monarch is related.
Monarchies often go hand in hand with religion. If a nation has a state religion, the monarchy will usually be strongly affiliated with it. This harks back to monarchy's roots, where the monarch claimed divine right -- his authority to run his country was granted by none other than God himself.
A monarch will rule for as long as the monarchy exists. Most politicians have term limits and, if not, have to be regularly re-elected. Monarchs, on the other hand, are appointed when a place opens (such a Prince's father dying), and will keep their position until they die, are deposed or, in rare circumstances, choose to abdicate.
As with most political systems, monarchies are not created equal. Rather, each monarchy is subject to its country's culture, social mores and political climate. Most 21st-century monarchies are constitutional monarchies, where the monarch's power is limited by a constitution. This means that the constitution is a higher authority than the monarch's. Constitutional limitations tend to be severe, too -- monarchs seldom play a substantial role in policy. On the other end of the spectrum, though, are absolute monarchies where the monarch is completely in charge. There is no constitution or legislative body, but rather just the monarch and his advisers who set policy without any checks to the king's or queen's authority.
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