IN BRITAIN'S "first-past-the-post" electoral system, seats won at a general election are not shared out between the parties proportionally nationwide. Instead each one of the 650 constituencies is self-contained, meaning any vote not used to win a seat is in effect wasted. In 2010 over 900,000 people voted for the populist UK Independence Party at the general election. They needn't have bothered: UKIP didn't win a single seat in the House of Commons. The Liberal Democrats (the best-known loser from this majoritarian system) joined the Tories in a coalition government after the 2010 election, but over 5.5m of its 6.8m votes made no contribution to its 57 seats. Under a proportional system its votes would have translated to 150 seats. At the 2015 election the Scottish National Party became the first minor party to win a number of seats in Westminster that outweigh its share of the popular vote.

Our interactive diagram above shows the outcome of general elections since 1997, including the 2015 result. The ring segments signify each party's share of the vote. Each filled square denotes a seat won by the party; each empty square shows additional seats it would have gained under a directly proportional system. Seats outside the ring represent those obtained beyond proportional allocation.


For more election graphics, visit our "data hub" here.