An octave is eight notes. Pitches that are an octave apart are twins of each other, sounding like the same note, just higher or lower.
To represent a pitch that is an octave higher, the letter is followed
by a prime (typed as a single quote
' next to the Enter key). To
represent notes that are an octave lower, the letter is followed by a
subprime (typed as a comma
play "C,, C, C C' C''"
Very high and very low pitches can be represented by repeating primes and subprimes.
Another way of writing higher pitches is to use lowercase letters.
ABC notation follows the Helmoltz pitch notation convention of assigning
lowercase letters to pitches one octave higher than uppercase letters.
So the pitch
c is the same as
C': one octave higher than middle
play "C c C c"