Like octagon and octopus, the key to an octave is “eight.” An octave may be the entire eight-note scale, or just the bottom and top notes, like C and C. Octave can also refer to an eight-line group in an Italian sonnet, or an eight-day festival; in fact, the word comes from the Latin expression octava dies, meaning “eighth day” — as in the eighth festival day after a feast. The ave in octave is usually pronounced “iv.”