"Countries with an electoral system based on proportional representation and party lists may introduce temporary special measures that would promote not only a high proportion of women candidates, but also a rank-order rule, such as a "zipper" system, where male and female candidates alternate, or where one of every three candidates through the list is from the less represented gender. Rank-order rules of this type remove the risk that women will be placed too low on party lists to have a genuine chance of being elected. It is also advisable to promote that if a female candidate withdraws her candidature, she is replaced with another woman."
OSCE (ODIHR), CoE (Venice Commission): Guidelines on Political Party Regulation, Second Edition, para. 188

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Guidelines on Political Party Regulation, Second Edition