The rationale underpinning all PR systems is to consciously reduce the disparity between a party's share of the national vote and its share of the parliamentary seats; if a major party wins 40 per cent of the votes, it should win approximately 40 per cent of the seats, and a minor party with 10 per cent of the votes should also gain 10 per cent of the legislative seats. This congruity between a party’s share of the vote and its share of the seats provides an incentive for all parties to support and participate in the system.
PR requires the use of electoral districts with more than one member: it is not possible to divide a single seat elected on a single occasion proportionally. There are two major types of PR system—List PR and Single Transferable Vote (STV). Proportionality is often seen as being best achieved by the use of party lists, where political parties present lists of candidates to the voters on a national or regional basis, but preferential voting can work equally well: the Single Transferable Vote, where voters rank-order candidates in multi-member districts, is another well-established proportional system.
There are many important issues which can have a major impact on how a PR system works in practice. The greater the number of representatives to be elected from a district, the more proportional the electoral system will be. PR systems also differ in the range of choice given to the voter—whether the voter can choose between political parties, individual candidates, or both.
