American Standard Pitch Notation

American Standard Pitch Notation (ASPN) identifies every pitch uniquely by combining a letter name with an octave number. The octave number changes at C: C4 is middle C, C5 is the C one octave above, and C3 is the C one octave below. The sequence runs C, D, E, F, G, A, B within each octave number.

Accidentals attach to the letter before the number: C♯4 is one half step above C4, and B♭3 is one half step below B3. Enharmonic equivalents like C♯4 and D♭4 denote the same physical key but are spelled differently depending on harmonic context.

ASPN is invaluable for clear communication about specific pitches across instruments and registers. When a score calls for a particular note, ASPN removes ambiguity — "play A4" means concert A 440 Hz, regardless of the instrument or clef in use. Musicians working in acoustics, electronic music, and music theory all rely on this standard.