In single-member constituencies

By-elections are held in most nations that elect their parliaments through single-member constituencies, whether with or without a runoff round. This includes most Commonwealth countries, such as the United Kingdom, Canada, and Pakistan, as well as non-Commonwealth countries such as France. In the United States they are called special elections, and are held when a seat in Congress, a state legislature or at the local level has become vacant.

In multi-member constituencies

When one seat in a proportional representation constituency becomes vacant, the consequences vary. For example, a by-election may be held to fill just the vacancy or all the seats in the constituency become up for grabs in the by-election held.

Scotland and New Zealand still hold by-elections, despite having adopted the additional member system, in which members are also chosen by party lists. The Republic of Ireland holds by-elections despite electing members in multi-member constituencies by the single transferable vote.

Alternatives to holding a by-election include having the most-voted losing candidate for the previous election fill the vacancy (excluding disqualified persons), as in Tasmania or the Australian Capital Territory, keeping the seat vacant until the next general election or nominating another candidate with the same affiliation as the one whose seat has become vacant – typically, in list systems, the next candidate on the party list. For the Australian Senate (where each State forms a multi-seat constituency voting by single transferable vote), the State Parliament appoints a replacement; however, in 1977 a referendum amended the Constitution to require that the person appointed must belong to the same political party (if any) as the Senator originally elected to that seat.

Consequences

The vast majority of by-elections are unimportant and voter turnouts are seldom comparable with general elections. The governing party normally has a solid cushion so that losing a handful of seats would not affect their position. Because by-elections usually have little influence on the general governance, voters feel freer to elect smaller fringe parties. Parties on both the far right-wing and the far left-wing tend to do better in by-elections than in general elections.

However, by-elections can become crucial when the ruling party has only a small margin. In parliamentary systems, party discipline is strong enough so that the one common scenario for a vote of no confidence to occur is after the governing party loses enough by-elections to become a minority government. A UK example was the Labour government of James Callaghan 1976-79. In the United States, Scott Brown's election in 2010 ended the filibuster-proof majority formerly enjoyed by Democrats. However, because a vote to end a filibuster requires a supermajority, the Democrats retained a majority of seats.

By-elections can also be important if a minority party needs to gain one or more seats in order to gain official party status or the balance of power in a minority or coalition situation. For example, Andrea Horwath's win in an Ontario provincial by-election in 2004 allowed the Ontario NDP to regain official party status with important results in terms of parliamentary privileges and funding.