A# / Bb

A#/Bb sits one semitone above A. As Bb it is one of the most important keys in jazz, brass, and wind music — trumpets and clarinets are typically Bb instruments. Bb major sounds warm and rounded, the horn-section key, while Bb minor is heavy and brooding — the key of Chopin's Funeral March.

Bb major is the home key of the trumpet and clarinet family. Much of the jazz repertoire — from swing to bebop — is written in Bb or closely related keys. The enharmonic spelling A# appears more often in sharp-key contexts like F# major.