The municipality consists of the following districts: Braine-l'Alleud (including the hamlet of Sart-Moulin), Lillois-Witterzée, and Ophain-Bois-Seigneur-Isaac.

The Duke, however, still owned exempt land (or franchise) on this territory, as specified in a legal document by Henry I dated 1197.

The name of the municipality changed to the current one, derived from "Braine", former name of the stream that crosses its territory (now called the "Hain"), and "alleu(d)", a medieval French word designating exempt land (English allod).

By 1489, however, the local lord enjoyed complete juridical power on its territory, which was still formally part of the fiefdom obtained from the Duke of Brabant.

The free-to-play, 18-hole Parc du Bourdon disc golf course is situated 1 kilometer from Braine L'Alleud train station.