Optional and mandatory referendums explained
Constitutional changes always go to a vote; laws only on request. That is the difference between a mandatory and an optional referendum.
Mandatory referendum
These must go to the people: changes to the Federal Constitution, joining organisations for collective security or supranational communities, and urgent federal acts without a constitutional basis that last longer than one year (Art. 140). They need the double majority of people and cantons, including urgent acts without a constitutional basis.
Optional referendum
A referendum can be requested against federal laws and certain other decisions within 100 days of publication. It needs 50,000 signatures or the request of 8 cantons (Art. 141). Certain international treaties are also subject to it.
What majority is needed?
For an optional referendum the popular majority alone decides. If no referendum is requested, the law takes effect without a vote.
The difference
Mandatory: the vote always takes place, usually with the double majority. Optional: the vote takes place only if 50,000 eligible voters or 8 cantons ask for it, and the popular majority is enough. An initiative, by contrast, comes from voters themselves.
Sources: Federal Chancellery (ch.ch, admin.ch), Federal Constitution Art. 140, 141, 141a.
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