In the event of the death of one of them, a by-election is held in which other hereditary peers belonging to the same party as the deceased can stand.

BBC: Viscount Ridley

It is not clear when a by-election will be held but the prospect will be an unwelcome one for Gordon Brown, left reeling by the expenses scandal and the spate of cabinet resignations in the past few days.

BBC: MP quitting to force by-election

Save that it is largely free from religious sectarianism, Paisley South is similar to nearby Monklands, where Labour only just held an even safer seat at a by-election in 1994.

ECONOMIST: By-election boulders