The government of the Roman republic is often described as "oligarchical." During the second half of the fifth century B.C.E., when Athenian ascendancy promoted democratic forms of government, there were still many oligarchic states in Greece, the most notable perhaps being at Corinth and at Thebes. In Greece, the oligarchs were often a section of the old nobility which had excluded the poorer nobles from power. Ancient Greek oligarchs characteristically possessed greater wealth and influence than the rest of the community, but noble birth was not a necessary condition for belonging to the ruling elite. Oligarchy (oligarchia, “rule by the few”) refers to the limitation of political power to only a small portion of the community, such as a few families or individuals (the oligarchs).
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The word oligarchy (l´ gärk) is derived from the Greek words for "few" (ὀλίγον, óligon) and "rule" (ἄρχω, arkho). In practice, almost all governments, whatever their form, are run by a small minority of members of society, and it is necessary to further examine the ways in which these individuals acquire and retain power in order to correctly understand whether a system of government is a oligarchy. Historically, there have been organized oligarchies, and unofficial oligarchies in which a group of “advisers” dictated the policies of an official ruler. However, neither wealth nor noble birth are necessary conditions for belonging to the privileged group which rules an oligarchy.
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Oligarchies have sometimes been synonymous with aristocracies, which were ruled by members of a noble class, or with plutocracies, ruled by the wealthy members of a community.