Party system

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A party system is a concept in political science concerning the system of government in a state where political parties exist.

The party system of a state determines how many political parties in a state have a realistic chance of becoming the government, usually through winning elections. Party systems are usually created by a variety of factors, such as the type of electoral system used in a state. For example, the use of First Past the Post in elections to a state's primary legislative assembly (for example general elections) usually results in a two-party system (this theory is called Duverger's law).

There are five distinct types:

Single-party system – when only one party can be the government because all other parties are banned;
Two-party system – when only two parties can realistically compete to become the government;
Dominant-party system – when only one party can realistically compete to become the government because of the weakness of other parties;
Multi-party system – when more than two parties can realistically compete to become the government.
No-party system – when no parties exist, because the constitution does not allow them. It is also called a non-partisan democracy.

There also exist two and a half party systems, for example in Germany until quite recently. In these there are two major parties, but neither is able to govern without the support of a third, smaller party. In the case of Germany, coalition governments from the 1960s until the late 1990s were invariably formed from either the SPD or the CDU/CSU (the two major parties) along with the support of the FDP.