In France, elections to the National Assembly, the French lower house, are known as élections législatives or simply les législatives. Although there are two chambers in the French system, the National Assembly has the last word on legislation and the upper house, the Senate, while it has considerable powers of delay and reflection, can only block legislation in a handful of areas, mostly those of a constitutional nature. Oddly enough, Senate elections are known as sénatoriales, but that's a story for another time.
Members of the National Assembly are known as députés or députées, and in French that word only refers to the elected office and never to a replacement or stand-in. The French National Assembly has, since the Second World War, had a five-year term. In its pre-war predecessor, the Chamber of Deputies, the term was four years.
Elections to the National Assembly are held on a single-member constituency basis, but over two rounds, one week apart. The election ends after the first round if any candidate achieves an absolute majority of votes cast in the first round, so long as that figure represents 25% of the total number of voters registered (not just those who turn out to vote) in the constituency.
As a rule, no more than 40 or 50 seats are decided in the first round. All candidates who have gained one eighth (12.5%) of the number of registered electors in a constituency can go forward to the second round, when a relative majority is enough for election.
When we look at French elections, we generally take the first round as the ‘genuine’ or authentic expression of political support in the country. But that support is exponentially distorted in the second round. This is deliberate. The founders of the Fifth Republic wanted a system that respected French electoral traditions (the same system was used most of the time under the Third Republic from 1871 to 1940) but which also delivered a stable majority, moving away from the instability created by the system of PR used under the Fourth Republic (1946-1958).
The system introduced in 1958, at the inception of the Fifth Republic has been used for all general elections since, except 1986, when the Socialist president François Mitterrand and his PM, Laurent Fabius, introduced single-round PR, in a cynical attempt to limit the expected right-wing majority. Despite their efforts, a coalition of right and centre-right parties took control of government and the Mitterrand was forced into a period of power-sharing, known as cohabitation.
A side-effect of the use of PR was that the Front National won 35 seats. Mitterrand’s critics, on the left and the right, blamed him for giving the far-right party credibility. The FN has never been anywhere near that number of seats since.
The législatives are subject to different pressures, both at national and local level. For example, parties sometimes strike national agreements that local parties opt to ignore... and vice versa. More often than not, the first round acts as a kind of primary. Obviously, candidates who have been eliminated cannot continue, but they well give their electors an idea of how they think they should vote. This is called the consigne de vote and we saw it after the first round of the presidential election, when several elminated candidates called on voters to choose Emmanuel Macron over Marine Le Pen.
Effectively, on the 11 and 18 June we will see 577 separate elections and in each constituency, candidates will do their sums and work out who is best placed to win the seat. It used to come down to a right vs left contest, but this time it will not be so straightforward. If you are reading about the elections in French, the key term is désistement. You will probably come across it most in the phrase désistement républicain, which refers to the other parties coming together to resist the Front National.