Some academic writers discuss states and governments in terms of "models".
An independent nation statenormally has a head of state, and determines the extent of its head's executive powers of government or formal

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Head of state is the highest-ranking constitutional position in a sovereign state. A head of state has some or all of the roles listed below,

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In a republic, the head of state nowadays usually bears the title of President, but some have or had had other titles. Titles commonly used by monarchs are King/Queen or Emperor/Empress, but also many other; e.g., Grand Duke, Prince, Emir and Sultan.

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Whenever a head of state is not available for any reason, constitutional provisions may allow the role to fall temporarily to an assigned person or collective body. In a republic, this is - depending on provisions outlined by the constitution or improvised - a vice-president

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In early modern Europe, a single person was often monarch simultaneously of separate states. A composite monarchyis a retrospective label for those cases where the states were

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Since antiquity, various dynasties or individual rulers have claimed the right to rule by divine authority, such as the Mandate of Heaven and the divine right of kings.

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• The polis in Greek Antiquity and the equivalent city states in the feudal era and later, (many in Italy, the Holy Roman Empire, the Moorish taifa in Iberia, essentially tribal-type but urbanised regions throughout the world in the Maya civilisation,

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