A berceuse [bɛʁsøz] is "a musical composition usually in 68 time that resembles a lullaby".
[1] Otherwise it is typically in triple meter.
Tonally most berceuses are simple, often merely alternating tonic and dominant harmonies; since the intended effect is to put a baby to sleep, wild chromaticism would be somewhat inappropriate.
Another characteristic of the berceuse, for no reason other than convention, is a tendency to stay on the "flat side"; noted examples including the berceuses by Chopin, who pioneered the form,[2] Liszt, and Balakirev, which are all in D♭.