When one seat in a multi-member constituency becomes vacant, the consequences vary. For example, a by-election may be held to fill just the vacancy, all the seats in the constituency could be contested in the by-election, or the vacancy could be filled by other means.

Seats which have unexpectedly changed hands in by-elections often revert to the former party in the next general election. One reason for this is that voter turnout at by-elections tends to be lower and skewed toward highly motivated supporters of the opposition party.