Bémol
French for the flat sign ♭.
What it means
Bémol is the French term for the flat sign, which lowers a note by a semitone; in Italian and Spanish the related forms bemolle and bemol are met with. The German technical term for it is B or Be. The word formation goes back to the medieval “soft B”, which stood opposite the “hard B” — from the latter developed, in the German-speaking area, the note name H as well as the signs for sharp and natural.
In the notation
The sign has the form of a small b with a round belly and stands to the left of the notehead. In the key signature it appears at the beginning of the line and holds for all notes of the same name in all octaves; as an accidental in the middle of the bar it affects only the notated octave position and loses its validity with the bar line. The order of the flats in the key signature follows the circle of fifths downward, beginning with B♭. Two such signs side by side form the double flat and lower by two semitones.
Musical use
The designation itself is met with above all in French-language editions, in key indications such as ré bémol majeur and in older theoretical literature. For practice, knowledge of it is useful above all in reading foreign-language music and work titles. In substance, everything holds that holds for the German B (flat): the sign creates the flat keys, makes leading tones and altered chord notes available, and, in combination with the natural sign, allows the taking-back of a lowering.
A source of error is the special German position of the note names: French si bémol corresponds to German b, that is English B♭, while French si corresponds to German H, that is English B. Whoever uses editions from both traditions side by side should keep this correspondence in mind.
Performance practice
If the term meets you in an edition, do not merely translate it but check at the same time whether the key designation too is meant differently than in German usage. With pieces from French or Spanish editions, note the key once in English spelling at the head of the first page — this spares the repeated conversion when learning.