The Additional Member System (AMS) of voting is used for elections to the Scottish Parliament, the National Assembly for Wales and the London Assembly.
Under AMS, voters are given two votes; one for an individual candidate and one for a party. Individual candidates are elected to single-member constituencies using first-past-the-post (or the second ballot or alternative vote). Under the second, party vote, representatives (additional members) are elected proportionally to a larger region.
The percentage of votes obtained by the parties in the party vote (second ballot) determines the overall number of representatives including those elected for the single member constituencies after taking into account the seats gained in each region by each party in the first ballot.
AMS therefore retains a constituency link while ensuring some proportionality. In elections to the Scottish Parliament there are 73 constituency members and 56 regional (additional) members. In the Welsh Assembly elections, there are 40 constituency members and 20 additional members.
Further information on this subject can be found from the following links.
The Electoral System for the Scottish Parliament
National Assembly for Wales website
London Assembly website