National Council seats are apportioned to the cantons based on their respective population size (which includes children and resident foreigners who do not have the right to vote).
As of the 2019 federal election, there are twenty multi-member constituencies with Zürich having the most number of National Council seats at 35.
Multi-member constituencies elect their National Council members by open list proportional representation, while single-seat constituencies elect theirs also by proportional representation but de-facto by first-past-the-post voting.
Proportional representation is used to elect Council of States members in Jura and Neuchâtel.
A federal election law designated the constituencies starting from 1851 until the introduction of the proportional representation system in 1919.