From MusiCAD

The octave in which a note is notated and/or played is indicated by a number after the note name.

A4 is the middle A on the piano (A4=440Hz) and the usual tuning tone.

To indicate that all notes should be played an octave higher than notation, use ottava (8va)

To move a note up an octave use <*>, move down an octave with </> musicad-octaven

note designation name              reach comments
C0 ''C sub-contra C0-B0 A0 is lowest note on piano, lower is sometimes found on a large pipe organ
C1 'C contra C1-B1
C2 C big C2-B2 Great because it used to be written with a capital letter
C3 c small C3-B3 Small because it used to be written with a small letter
C4 c' one lined C4-B4 C4 is middle C on piano as well as in notation on a piano system. Middle A is also in this octave
C5 c'' two-lined C5-B5
C6 c''' three-lined C6-B6 C6 is the 'high c' of a soprano voice
C7 c'''' four-lined C7-B7
C8 c''''' five-lined C8-B8 C8 is the highest note on a piano, much higher notes are not musically interesting (because inaudible for many people)

See also