Originally Posted by Wiki
By calling a federal referendum a group of citizens may challenge a law that has been passed by Parliament, if they can gather 50,000 signatures against the law within 100 days. If so, a national vote is scheduled where voters decide by a simple majority whether to accept or reject the law. Eight cantons together can also call a referendum on a federal law.
Similarly, the federal constitutional initiative allows citizens to put a constitutional amendment to a national vote, if they can get 100,000 voters to sign the proposed amendment within 18 months. Parliament can supplement the proposed amendment with a counter-proposal, with voters having to indicate a preference on the ballot in case both proposals are accepted. Constitutional amendments, whether introduced by initiative or in Parliament, must be accepted by a double majority of both the national popular vote and a majority of the cantonal popular votes.