Factbox: Swiss referendum may decide UBS tax deal

(Reuters) - A Swiss tax deal with the United States, crucial to the future of Swiss bank UBS, may be put to a referendum, after the lower house of parliament signed off on the deal.

Referendums are a key part of Switzerland's unique tradition of direct democracy and a central feature of political life.

Here are some details:

* WHAT IS THE DEBATE:

-- The debate is over whether to allow a facultative referendum on the Swiss-U.S. tax deal that would see the bank data of 4,450 UBS clients who dodged U.S. taxes delivered to Washington.

-- In Switzerland, referendums are held at federal, cantonal and municipal level. It is not the government's choice whether or when a referendum is held, but a legal procedure regulated by the Swiss constitution. There are three types of referendums:

* FACULTATIVE REFERENDUM:

-- Any federal law, certain federal resolutions, and international treaties that are perpetual or irredeemable, joinings of an international organization, or that change Swiss law may be subject to a facultative referendum if at least 50,000 people or eight cantons have petitioned to do so within 100 days of its coming into force.

-- Within cantons and municipalities, the required number of people is smaller.

-- The facultative referendum is the most common type of referendum, and it is mostly carried out by political parties or by interest groups.

* OBLIGATORY REFERENDUM:

-- It takes place at national level on any amendment of the constitution and decisions to join multinational community or an organization for collective security.

* CITIZEN'S INITIATIVE:

-- While the two previous types of referendums are votes on existing legislation, this type of referendum gives citizens the right to initiate law.

-- Citizen's initiatives at the federal level need to collect 100,000 valid signatures within 18 months, and must not contradict international laws or treaties.

(Writing by David Cutler and Christopher Barnett; London Editorial Reference Unit; editing by Simon Jessop)