The country is divided into 13 regions, ranging in size from the South East (ten seats) and London (eight seats) down to the North East and Northern Ireland (three seats each).
The main parties all select enough candidates to contest all of a region’s seats, while smaller parties might only contest some of the available seats.
The parties arrange their candidates on their list, so candidates that are placed at the top would win seats first if their parties get enough support.
The ballot paper shows each party’s list and voters chose just one party to support using a single X vote. All the votes in each region are then counted and each party get seats in proportion to the party’s vote share.
So, suppose we had a region with ten seats where party A got 40% of the vote – they should end up with four of the available seats. This system is proportional but it may favour larger parties if votes are heavily fragmented across many smaller parties.