Proportional representation (PR) characterizes electoral systems in which divisions in an electorate are reflected proportionately in the elected body. The concept applies mainly to geographical and political divisions of the electorate. For instance in the European parliament, each member state has a number of seats that is (roughly) proportional to its population, enabling geographical proportional representation. Almost all European countries also have political proportional representation (ideological proportional representation to the degree that parties honestly describe their goals): When n% of the electorate support a particular political party or set of candidates as their favorite, then roughly n% of seats are allotted to that party or those candidates. The essence of such systems is that all votes contribute to the result—not just a plurality, or a bare majority. The most prevalent forms of proportional representation all require the use of multiple-member voting districts (also called super-districts), as it is not possible to fill a single seat in a proportional manner. In fact, PR systems that achieve the highest levels of proportionality tend to include districts with large numbers of seats, as large as a province or an entire nation.The most widely used families of PR electoral systems are party-list PR, single transferable vote (STV), and mixed-member PR (MMP).
With party list PR, political parties define candidate lists and voters vote for a list. The relative vote for each list determines how many candidates from each list are actually elected. Lists can be "closed" or "open". Closed lists are determined before the elections, usually by the party heads or by primary elections. Open lists allow voters to indicate preferences for individual candidates during the elections.
With single transferable vote, voters can rank individual candidates, rather than just vote for a single "best" candidate. During the count, as candidates are elected or eliminated, surplus or discarded votes that would otherwise be wasted are transferred to other candidates in order of preferences, forming consensus groups that elect surviving candidates. STV enables voters to vote across party lines, to choose the most preferred of a party's candidates and vote for independent candidates, knowing that if the candidate is not elected their vote will likely not be wasted.
Mixed member PR (MMP), also called the additional member system (AMS), is a two-tier mixed electoral system, combining local non-proportional plurality/majoritarian elections and a compensatory regional or national party-list PR election. Voters typically have two votes, one for their single-member district and one for the party list. Parties that are under-represented by district elections are compensated by additional members, such that the total number of members of each party is proportional based on the party-list vote.According to the ACE Electoral Knowledge Network, some form of proportional representation is used for national lower house elections in 94 countries. Party list PR, being used in 85 countries, is the most widely used. MMP is used in seven lower houses. STV is used in only two: Ireland, since independence in 1922, and Malta, since 1921. STV is also used in the Australian Senate, and can be used for nonpartisan elections such as the city council of Cambridge MA.Due to factors such as electoral thresholds and the use of small constituencies, as well as manipulation tactics such as party splitting and gerrymandering, perfect proportionality is rarely achieved under these systems. Nonetheless, they approximate proportionality much better than other systems. Some jurisdictions use leveling seats to compensate for such factors.