A♯ / B♭

A♯ / B♭ is pitch class 10 — ten semitones above C, two below the octave. A♯ rises from A; B♭ falls from B. Same class in equal temperament; spelling follows the key or chord.

Chromatically it sits between A and B, near the top of the octave climb from C. It is a black-key class on the usual keyboard picture.

In major-triad grammar this class is the root of B♭ major (B♭–D–F), the major third of F♯ major (F♯–A♯–C♯), and the perfect fifth of E♭ major (E♭–G–B♭). Alone it stays a pitch class. Key character appears only when a major triad meets in the vessel.

Also known as A sharp · A♯ · B flat · B♭ · Bb · A#