It connects Herrmann-Debroux in the south-east of Brussels to Erasme/Erasmus in the south-west via the city centre.

Starting from Herrmann-Debroux, the line crosses the municipalities of Auderghem, Etterbeek, City of Brussels, Molenbeek-Saint-Jean, Koekelberg and Anderlecht.

It serves 28 metro stations and has a common section with line 1 between Gare de l'Ouest/Weststation and Merode, and with lines 2 and 6 between Gare de l'Ouest and Beekkant.

The first section of this line was built in the late 1960s between Schuman and De Brouckère, but was served by trams.

The line was also expanded westwards, to Sainte-Catherine/Sint-Katelijne in 1977, to Beekkant in 1981, to Bockstael in 1982, to Heysel/Heizel in 1985 and to Roi Baudouin in 1998.