This is a list of the 646 previous constituencies of the United Kingdom parliament. There are 650 in the 2010 election constituencies currently represented in the House of Commons of the Parliament of the United Kingdom, as at the 2005 general election. Each constituency is represented by a single Member of Parliament (MP). Constituency boundaries are subject to regular review by the four independent Boundary Commissions, usually once every 10 to 15 years, to keep the electorate of each constituency as close to the national average as is reasonably possible. New constituencies may be created, or existing ones abolished, by these reviews. For the list of recommended constituencies following the current review see Constituencies in the next United Kingdom general election. Constituencies were long based on boroughs (burghs in Scotland) and counties. Today, constituencies in England are mostly subdivisions of local authorities, with each constituency comprising a number of whole wards. In Scotland, constituencies are subdivisions of council areas, and in Wales they are subdivisions of the preserved counties. Northern Ireland is reviewed as a whole, and constituency boundaries may cross all district borders. In some cases, particularly in urban areas, two or more local government areas may be combined to form a single review area, so that particularly large or small constituencies are not created. For example, if two adjacent areas are entitled to 1.5 constituencies each, they may be combined and awarded three constituencies, rather than having two constituencies each, all of which would be well below the average constituency electorate. The average constituency size is approximately 74,000 registered voters, but they vary in size from the smallest (Na h-Eileanan an Iar at 22,200 voters) to the largest (the Isle of Wight at approximately 110,000 voters). A constituency has no physical size restrictions. The Parliament of 2001 contained representatives from 659 constituencies. Most of the current constituency boundaries were last reviewed in the early 1990s, and are therefore based on administrative boundaries prior to the last series of local government boundary changes. However, a Boundary Commission for Scotland review in February 2005 resulted in the reorganisation of most Scottish constituencies to adjust for the historic over-representation of Scotland. This reduced the number of constituencies in Scotland by 13, from 72 down to the current 59. The Parliament of 2005 therefore had 646 representatives. In the 2010 Election on 6 May 2010 the number of seats contested has increased