Zipper system

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The zipper system, also known as "vertical parity" or the "zebra system",^[1][2] is an electoral mechanism intended to enforce gender parity in countries using party-list proportional representation with closed lists. It requires that parties alternate between candidates of either gender on their candidate lists, meaning that 50% of the candidates on the list are women and 50% are men.^[3] The zipper system is applied to election laws with the goal of establishing gender parity.^[3]

However, the mathematical mechanism behind the naive zipper system is faulty, and as a result will favor whichever gender is more likely to be placed on the list first.^[4] More modern variants on the zipper system, based on the fair share sequence, biproportional apportionment, and other rules can enforce gender quotas much more precisely, while introducing less distortion into party lists.^[4]