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B♭ (B-flat; also called si bémol) is the eleventh step of the Western chromatic scale (starting from C).
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It lies a diatonic semitone above A and a chromatic semitone below B, thus being enharmonic to A♯ although in some musical tunings, B♭ will have a different sounding pitch than A♯. B-flat is also enharmonic to C.
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. See pitch (music) for a discussion of historical variations in frequency.
While orchestras tune to an A provided by the oboist, wind ensembles usually tune to a B-flat provided by a tuba, horn, or clarinet.
In Germany, Russia, Poland and Scandinavia this pitch is designated B, with 'H' used to designate the B-natural (or enharmonically C♭). 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. See B (musical note) for an explanation.