Under the AV system, electors rank candidates in order of preference.

BBC: Kennedy: 'Campaign against Tories'

The AV system allows electors to rank candidates in order of preference.

BBC: Referendum on voting system goes ahead after Lords vote

The AV system asks voters to rank candidates in order of preference.

BBC: Q&A: Alternative vote referendum

The bill paves the way for a referendum on changing the Westminster electoral system from first-past-the-post to the Alternative Vote (AV) system, whereby voters rank candidates in order of preference on the ballot paper.

BBC: Parliamentary Voting System Bill part two

The election is taking place under an alternative vote system which allows party members to rank candidates in order of preference.

BBC: Wales politics

Roth and his team were able to weave all that complexity into one transparent, reliable system where students list up to 12 choices in order of preference.

FORBES: What Al Roth Did To Win The Nobel Prize In Economics