Of the 129 MSPs, 73 are elected to represent first past the post constituencies and are known as "Constituency MSPs". Voters choose one member to represent the constituency, and the member with most votes is returned as a constituency MSP. The 73 Scottish Parliament constituencies shared the same boundaries as the UK Parliament constituencies in Scotland, prior to the 2005 reduction in the number of Scottish MPs, with the exception of Orkney and Shetland which each return their own constituency MSP. Currently, the average Scottish Parliament constituency comprises 55,000 electors. Given the geographical distribution of population in Scotland, this results in constituencies of a smaller area in the Central Lowlands, where the bulk of Scotland's population live, and much larger constituency areas in the north and west of the country, which have a low population density. The island archipelagos of Orkney, Shetland and the Western Isles comprise a much smaller number of electors, due to their dispersed population and distance from the Scottish Parliament in Edinburgh. If a Constituency MSP resigns from Parliament, this triggers a by-election in his or her constituency, where a replacement MSP is returned from one of the parties by the plurality system.