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:
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.