Octave
An octave is the distance from a note to the next note with the same name, for example from one C to the next C. It spans eight scale notes and twelve half steps.
The word comes from the Latin for “eight”: counting the white keys from C to the next C, C D E F G A B C, gives eight notes. Counting every key, black and white, gives twelve half steps. The octave is the largest interval beginners learn.
A note an octave higher vibrates exactly twice as fast, so the two notes blend and sound “like the same note”. That is why they share a name, and why the keyboard is divided into octaves, each running from C up to B. The octaves are numbered: middle C is C4, the next C up is C5.
On the piano an octave fits the hand: most adults can reach from C to C with the thumb and little finger. The left hand often plays the bass in octaves.
What it sounds like
Two notes an octave apart sound so alike that, played together, they merge into one fuller sound. You can hear an octave leap at the start of “Somewhere over the Rainbow”.
Where it is taught: the studio’s interactive textbook, chapter 1, “The keyboard and your first notes”, chapter 5, “Half steps, whole steps and the black keys” and chapter 15, “Intervals: unison to octave”.