New members of parliament generally assume office during the parliamentary session following their election. Their mandate is for four years and can be extended if they successfully stand for re-election.
National Council
If a member of the National Council steps down during these four years (the legislature period):
if the canton has more than one seat in the National Council, the new member of parliament takes up office at the start of the following session of parliament.
if the canton only has one seat in the National Council, a new election is called in the canton to elect a successor.
Council of States
If a member of the Council of States steps down, a new election is held in the canton to elect a successor.