B♭ (B-flat), or, in some European countries, B, is the eleventh step of the Western chromatic scale (starting from C). It lies a diatonic semitone above A and a chromatic semitone below B,^[1] thus being enharmonic to A♯, even though in some musical tunings, B♭ will have a different sounding pitch than A♯. B-flat is also enharmonic to C (C-double flat).

When calculated in equal temperament with a reference of A above middle C as 440 Hz, the frequency of the B♭ above middle C is approximately 466.164 Hz.^[2] See musical pitch for a discussion of historical variations in frequency.

While orchestras tune to an A provided by the oboist,^[3] wind ensembles usually tune to a B-flat provided by a tuba, horn, or clarinet.^{[citation needed]}

In Germany, Russia, Poland, Scandinavia and Slovakia this pitch is designated B, with 'H' used to designate the B-natural. Since the 1990s, B-flat is often denoted Bb or "Bess" instead of B in Swedish music textbooks. Natural B is called "B" by Swedish jazz and pop musicians, but still denoted H in classical music.^{[citation needed]} See B (musical note) and Note names and their history for explanations.