Note of the scale (6th degree of natural scale of C). Hence A♭, A♭♭, A♮, A♯, A♯♯, A major, A minor, etc. A is note commonly used for tuning instr. (orchs. tune to the ob. A). a′ = 440 vibrations per second, internationally accepted since 1939, although some orchs. still accept a′ = 435 and (in USA) a′ = 445.