Two-party system
There are several states in the world, where politics is dominated by two parties. This is known as two-party system. According to Giovanni Sartori, the following is true for such as system:
Each of the two parties is able to reach an absolute majority in parliament
When there are elections, usually one of the two parties wins an absolute majority
The party that is elected wants to rule alone
There's a large probability that with elections, the ruling party will change over time
The Median voter theorem describes such a system: While at first the parties have programmes that are very different from each other, over time, they will get closer. Hotelling's law also describes this in game theory.
Note that in such systems, there may also be other parties, bur these rarely have enough influence to win majorities in elections.
Two Examples of countries that have a two party system are the United Kingdom, and the United States.
Related pages
Multi-party system - A system where there are many parties, Sometimes, several parties work together in government.
One-party state - There's only one party, but sometimes people can choose from different candidates of that party.
Political systems