section_path: Party systems
section_name: Party systems
paragraph_text: Party systems
  
  Political parties are ubiquitous across both democratic and autocratic countries, and there is often very little change in which political parties have a chance of holding power in a country from one election to the next. This makes it possible to think about the political parties in a country as collectively forming one of the country's central political institutions, called a party system. Some basic features of a party system are the number of parties and what sorts of parties are the most successful. These properties are closely connected to other major features of the country's politics, such as how democratic it is, what sorts of restrictions its laws impose on political parties, and what type of electoral systems it uses. Even in countries where the number of political parties is not officially constrained by law, political institutions affect how many parties are viable. For example, democracies that use a single-member district electoral system tend to have very few parties, whereas countries that use proportional representation tend to have more. The number of parties in a country can also be accurately estimated based on the magnitude of a country's electoral districts and the number of seats in its legislature.
  
  An informative way to classify the party systems of the world is by how many parties they include. Because some party systems include a large number of parties that have a very low probability of winning elections, it is often useful to think about the effective number of parties (the number of parties weighted by the strength of those parties) rather than the literal number of registered parties.