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. Non-partisan systems In a non-partisan system, no political parties exist, or political parties are not a major part of the political system. There are very few countries without political parties. In some non-partisan countries, the formation of parties is explicitly banned by law. The existence of political parties may be banned in autocratic countries in order to prevent a turnover in power. For example, in Saudi Arabia, a ban on political parties has been used as a tool for protecting the monarchy. However, parties are also banned in some polities that have long democratic histories, usually in local or regional elections of countries that have strong national party systems. Political parties may also temporarily cease to exist in countries that have either only been established recently, or that have experienced a major upheaval in their politics and have not yet returned to a stable system of political parties. For example, the