Octaves
Walk twelve semitones up from any key and you land on the same letter again. That distance is an octave. The higher note vibrates exactly twice as often: double the frequency, same letter.
Four A's, four octaves
Every octave doubles a note's frequency. The note A appears at 55, 110, 220, and 440 Hz, each one an octave above the last.
The number in a note's name (A2, A3, A4) tells you the octave. Higher number, higher pitch. A new number starts at every C: after B3 comes C4.
Same note, higher up
An octave sounds like the same note at a higher or lower pitch.
Play one C with the next C: they blend into one sound. Play C with D: two separate notes. That blend is why both keys get the same letter.
Why an octave is called an octave
Octave is Latin for eighth, the same oct- as in octopus. Count up the C major scale and the eighth note is C again.
The name counts letter names, not semitones. An octave spans twelve semitones but eight letter names.
Game · Sing an octave
1 / 2Sing the A in any octave that fits your voice, then sing the A an octave up.
Hear a note, sing it, then sing the same letter an octave up. It's the leap that opens "Somewhere Over the Rainbow". Start low so you have room.
Quiz
1 / 4A4 vibrates 440 times a second. What's A5?