Substitute (elections)
A substitute is a political candidate who is not directly elected, but who succeeds a politician holding an elected office after that person ceases to hold the office due to, for example, resignation or death. This system can be used to fill casual vacancies instead of holding by-elections or special elections to fill the vacant office. Substitutes are nominated, not at the time the vacancy arises but, rather, before the election for the information of voters. In voting systems which use electoral lists, the candidates on a given list who are not among those initially elected may become the substitutes for those who are. In other systems, individual candidates may have substitutes.
A substitute is a political candidate who is not directly elected, but who succeeds a politician holding an elected office after that person ceases to hold the office due to, for example, resignation or death. This system can be used to fill casual vacancies instead of holding by-elections or special elections to fill the vacant office. Substitutes are nominated, not at the time the vacancy arises but, rather, before the election for the information of voters. In voting systems which use electoral lists, the candidates on a given list who are not among those initially elected may become the substitutes for those who are. In other systems, individual candidates may have substitutes.

Belgium
In Belgian federal parliamentary elections, each electoral list has both a list of "effective" candidates and a list of "substitutes" (Dutch: opvolgers; French: suppléants). The system was introduced as part of the law of 29 December 1899 introducing proportional representation. Before that, by-elections were held to succeed members.
For municipal and provincial elections, as well as those for the Parliament of the Brussels-Capital Region since 2019, there is only one list, and substitutes are designated, according to various systems from one region to another, on the base of their own preferential votes, weighted or not with list votes, i.e. not for one or several specific candidate(s).

France
In the elections for the French National Assembly, each candidate nominates a substitute  (French: Suppléant), who assumes the functions of the elected deputy if they die, enter the executive government, if the Government appoints them to an assignment of more than six months' duration, or if they are appointed to the Constitutional Council or Defender of Rights (Défenseur des droits).If the deputy resigns, or their election is determined to be invalid, a by-election (French: élections legislatives partielles) is held instead.
The Electoral Code does not provide for any age restriction to be appointed alternate. For the Fourteenth Legislature (2012 - 2017), the youngest Deputy-Substitute in France was Nicolas Brien, born in 1989, who was elected in Allier's 2nd constituency.

Examples
2017Élise Fajgeles replaced Benjamin Griveaux when he was appointed Secretary of State to the Minister of the Economy and Finance on 22 July 2017.
Grégory Galbadon replaced Stéphane Travert when he was appointed Minister of Agriculture on 22 July 20172018Jean-Louis Thiériot replaced Yves Jégo in Seine-et-Marne's 3rd constituency.2019Stéphanie Atger replaced Amélie de Montchalin when she was appointed Minister of Europe and Foreign Affairs.2020Sandra Boëlle replaced Claude Goasguen in Paris's 14th constituency when he died.
Nicolas Meizonnet replaced Gilbert Collard in Gard's 2nd constituency when he was elected to the European Parliament.2021Maud Gatel replaced Marielle de Sarnez in Paris's 11th constituency when she died


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