FACTBOX: Key facts about Belgium's electoral system

(Reuters) - Belgians voted on Sunday in a general election.

Here are some key details on the country's electoral system.

ELECTORATE: About 7.7 million of Belgium's population of 10.5 million. It is compulsory for all people aged 18 or over to fill in a ballot sheet.

ELECTORAL SYSTEM: Proportional representation, with 5 percent minimum threshold.

PARLIAMENT STRUCTURE: The parliament is split into the Chamber of Representatives (150 seats) and the Senate (71 members of which 40 are directly elected). The Chamber is the more powerful of the two, with control over state finances.

VOTING: Two votes -- one for the Chamber, the other for the Senate. Voters are given a list of candidates from each party and can either vote for this list, backing the party's order on who should get seats, or choose specific candidates from it, potentially pushing that person up the pecking order.

CONSTITUENCIES: For the Chamber, parties have lists for each of Belgium's 10 provinces and the Brussels region. The number of seats available depends on each province's population.

For the Senate, there are just two lists -- one for the Dutch-speaking Flemish community and one for the French-speaking region of Wallonia. Residents of Brussels, the third community, choose from one of the two lists. Belgium's tiny German-speaking community select from the Walloon list.

While the Chamber has more power, politicians seeking to become prime minister are typically placed as the leading candidate for the Senate because it is a clearer demonstration of their popularity in their respective communities. The Senate vote is effectively a prime ministerial popularity contest.

GOVERNMENT FORMATION: After the election, the monarch typically appoints a senior politician (informateur) to investigate whether potential political coalitions are viable. The informateur can be replaced.

The monarch subsequently appoints a person (formateur) to form a new government. This person will typically be the next prime minister. The monarch can immediately designate someone as the formateur, without the need for an informateur.

The entire process can take from a few weeks to a few months. It is expected to last at least two months this time.

The government is in power for four years.