The octave

We touched on this in chapter 2: after G Sol , the alphabet wraps back to A La . The next A La sounds higher than the first, but it carries the same name. The distance between those two same-named notes is called an octave.

The word comes from Latin octava — “eighth.” If you count up from the starting note through the alphabet (A, B, C, D, E, F, G, A) the eighth note you reach is the next octave.

What an octave really is

The reason two notes an octave apart share a name is not convention. It’s acoustics.

When a note vibrates at 440 Hz (cycles per second), the note an octave above it vibrates at exactly 880 Hz — double. The note an octave below vibrates at 220 Hz — half. This doubling-and-halving pattern is the fundamental ratio of musical pitch, and our ears are wired to perceive it as sameness.

Play a C Do on the keyboard below, then play the C Do one octave above it. They are different pitches — one is clearly higher — but they share a quality that feels like the same note:

The three C Do keys are highlighted. Click them in turn and listen.

Octave naming

To tell octaves apart on paper, musicians attach a number to the letter. Middle C on a piano — the C in the middle of the keyboard — is called C Do 4. The next C up is C Do 5, the next down is C Do 3. The bouzouki’s open D string (the highest) is D4.

You won’t see these numbers often in chord charts or basic notation — they matter most when you need to be precise about which exact octave a note sits in. For now, just know that the system exists.

Recap

• An octave is the distance between two notes that share the same name but sit at different heights — like a low C Do and a high C Do .
• Acoustically, an octave is a doubling of frequency. 440 Hz to 880 Hz is one octave.
• Octaves are why the musical alphabet wraps. Every letter recurs again and again, higher and higher.