Supplementary Member

Parallel voting describes a mixed electoral system where voters in effect participate in two separate elections for a single chamber using different systems, and where the results in one election have little or no impact on the results of the other.

Specifically, it usually refers to the semi-proportional system used in Japan, South Korea, Taiwan, some regions of Russia and elsewhere, sometimes known as the Supplementary Member (SM) system or, by some political scientists, Mixed Member Majoritarian (MMM), which combines first-past-the-post voting (FPTP) with party-list proportional representation (PR). Parallel voting or MMM is distinct from mixed-member proportional representation (MMP) where a single election takes place, and the party vote determines what share of seats each party will receive in the legislature to "top up" its constituency seats.

Whilst FPTP-PR is the most common pairing in parallel systems, any other combination is effectively possible. For example, in Italy and France, regional elections are held under a parallel system where a group of councillors are chosen by a party-list system, and the remaining part with a general ticket, so to ensure that a single list wins well over half the seats.

Under a supplementary member system (SM), which is a form of semi-proportional representation, a portion of seats in the legislature are filled by pluralities in single-member constituencies. The remainder are filled from party lists, with parties often needing to have polled a certain quota, typically a small percentage, in order to achieve representation, as is common in many proportional systems. Any supplementary seats won by a party are filled from an ordered list of nominated candidates

Unlike mixed-member proportional, where party lists are used to achieve an overall proportional result in the legislature, under SM, proportionality is confined only to the list seats. Therefore, a party that secured say 5% of the vote will have only 5% of the list seats, and not 5% of all the seats in the legislature.

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