The Northern Ireland Assembly has a "seat-switch system" (co-option) that allows for MLAs to be replaced without holding by-elections.

As the BBC explains:

Co-option is the only option when there is a gap on the benches of the Northern Ireland Assembly.

Some Assembly members leave Stormont mid-term for a variety of reasons, and their party selects the successor.

Are there any legislatures that have similar co-option systems that allows for replacements of members without holding by-elections?

3 Answers 3


This is the typical process in a system based on proportional representation and a party list. For example using the D'Hondt method of apportion.

For example, in the Netherlands, Andeweg and Irwin state that "If a vacancy occurs it is filled by beginning at the top of the list and continuing down until a candidate on the list accepts election."

  • Is the restriction to single member constituencies necessary? I understand block plurality voting to still require by-elections to replace vacancies.
    – origimbo
    Jan 12, 2020 at 14:35
  • I think the list system is the important part. I'm not sure the last sentence is accurate, although I'm not willing to do the research required to determine if 'normally' gives enough wiggle room. Republic of Ireland is PR based and does have by-elections. It does not use the list system.
    – Eric Nolan
    Jan 14, 2020 at 10:15
  • To clarify, the Dutch system requires a party to decide up front on the list order. After the elections, the order is fixed, even if the people would leave the party (!). This becomes relevant when a party fractures due to an internal dispute; under the Dutch system there's no need to answer which fraction is the legitimate successor.
    – MSalters
    Jan 16, 2020 at 16:43

In the Australian Senate casual vacancies are filled by the respective state or territory government choosing a replacement from the party of the old senator at the time of their election. (If they have changed party allegiances during their term, the replacement will not be from the new party.) If the state parliament is not in session then the state governor will appoint the replacement provisionally, to be confirmed in the parliament's next session.


There is in Switzerland

In a few Swiss Cantons (Genva, Grisons, Jura, Neuchâtel and Valais) there is a system of "substitues" deputies (members of cantonal parliament). The system is different in each Canton, but substitues are usually (GE, JU, NE and VS) from the same electoral list as the deputies they could replace.

Neuchâtel's Law on Political Rights defines it in articles 63a to 64.

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