Octaves and register

Play a C, then play the next C above it. The second note is clearly higher, and also clearly the same note. That relationship is the octave, and it is the strongest relationship in music.

Why they sound the same

The upper note vibrates at exactly twice the frequency of the lower one. A note at 220 Hz and a note at 440 Hz are both A. Double it again to 880 Hz and it is still A.

That doubling is the simplest possible frequency relationship, and human hearing treats notes an octave apart as versions of one another — a phenomenon usually called octave equivalence. It is why the musical alphabet can stop at seven letters and start again.

Octave numbering

Because every C is called C, you need a way to say which C. The common system is Scientific Pitch Notation, which numbers octaves from C upwards:

• C4 is middle C.
• A4 is the A above it — the 440 Hz tuning reference.
• C5 is an octave above middle C; C3 is an octave below.

The number changes at C, not at A. So B3 and C4 are adjacent notes with different octave numbers.

Register

Register is the loose word for which part of the range you are in — low, middle, high. It is less precise than octave numbering and more useful in conversation: a melody "in a high register" or a "low, muddy register" says something about character rather than exact pitch.

Instruments have registers with their own names and their own sound. The same written note can feel completely different depending on the register it lands in.

Where this shows up

Reading music: the staff covers a limited range, and notes outside it need ledger lines. Middle C is the pivot between the treble and bass staves precisely because it sits in the middle of the register most instruments share.

Harmony: a chord's notes can be spread across octaves in any arrangement without changing what chord it is. That is why inversions rename the bass note rather than the chord.