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

Assuming a presidential election is held in June, and that Mr Karzai wins, he could be obliged to rule by decree for almost a year.

ECONOMIST: Can the elections be held on time?

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

In other words, if another election is held, he could turn the choosing of the next chief executive into a genuine popular vote by vastly expanding the sector electorates to include virtually every working person in Hong Kong.

CNN: From Our Correspondent: Why the Confusion?