The Additional Member System (AMS), part of the proportional representation systems that exist in elections, has been used with the sphere of British Politics. The Additional Member System has been used in devolutionary elections in Britain but is not yet seen as a replacement for the first-past-the-post system used in Britain's national elections.
AMS is considered to be a a hybrid electoral system.
The first vote is for a constituency member and the result of this is decided on a FPTP system
The second vote is for a party and some of the seats are awarded on a list system.
The number of seats on the ‘top up’ list system varies from 25% to 50% of the total available.
The lists can be either regional or national.
Voters have two choices.
Small parties have a theoretical advantage as a result of the system but it rarely manifests itself in favour of the small parties.
All votes count in the second vote for the party.
AMS does not usually produce governments with a decisive majority (see the Welsh Assembly)
Coalition governments are common but with AMS such governments are dominated by one party.