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* Hệ thống này được dùng ở các nước như [[Australia]], [[Cộng hòa Ireland]], [[Bắc Ireland]], [[Scotland]], và [[New Zealand]].
== Tính tỷ lệ một phần ==
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Some nations with proportional elections, like Israel and the Netherlands, have one electoral district only: the entire nation, and the entire pie is cut up according to the entire outcome. Most nations have district systems in place where more than one person is elected per district. The constituency or district magnitude (DM) of a system is therefore measured by the number of seats in a constituency, and plays a vital role in determining how proportional an electoral system can be. The greater the number of seats in a constituency, the more proportional the outcome will be. PR applied to a single-member district (SMD) is by necessity majoritarian. If the constituency is in a jurisdiction using list PR in its multi-member districts (MMDs) the winning candidate simply needs a [[plurality]], otherwise called a simple or relative majority, of the vote to win, so that the election in the SMD is by [[first-past-the-post]]. If the constituency is in a jurisdiction using PR-STV in its MMDs, an absolute majority of 50% plus 1 will likely be the minimum required for victory (depending on which quota is used) so that the election in the SMD is by the [[alternative vote]]. Four elected officials per district delivers a threshold of 20% (1/M+1) to gain a single seat. However, constituency borders can still be [[gerrymander]]ed to reduce the overall proportionality. This may be achieved by creating "majority-minority" constituencies - constituencies in which the majority is formed by a group of voters that are in the minority at a higher level. Proportional representation with the entire nation electing the single body, cannot be gerrymandered.
Multiple-member districts do not necessarily ensure that an electoral system will be proportional. The [[bloc vote]] can result in "super-majoritarian" results in which geographical variations can create majority-minority districts that become subsumed into the larger districts. Also, in theory, a party, who does not provide a list with enough people to fill all the seats won by it, may be given those unfilled seats. This is termed an [[underhang seat|underhang]].
Some nations, with either exclusively proportional representation or — as is the case with Germany — [[additional member system]]s, require a [[party list]] to achieve an [[election threshold]] — a certain minimum percentage of votes to receive any seats. Typically, this lower limit is set at between two and five percent of the total number of votes cast. Parties who do not reach that margin will not be represented in parliament, making majorities, coalitions and thus governments easier to achieve. Proponents of election thresholds argue that they discourage excessive fragmentation, [[balance of power#Parliamentary politics|disproportionate power]], or extremist parties. Opponents of thresholds argue that they cause unfair redirection of support from minor parties, thus giving the parties which cross the threshold disproportionally high percentages of the seats and creating the possibility that a party or group of parties will assume control of the legislature without gaining a majority of votes.
There are several ways of measuring proportionality, the most common being the [[Gallagher Index]].
== Hệ thống đa đảng và hệ thống tỷ lệ trung tâm ==
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