Octaves: middle C is C4 · Notes and the staff · Music theory
To say which C or which A you mean, each octave has a number. The octave starts on C, so B3 and C4 are neighbors. Middle C, the one in the middle of the…
Octaves: middle C is C4
To say which C or which A you mean, each octave has a number. The octave starts on C, so B3 and C4 are neighbors. Middle C, the one in the middle of the piano, between the treble and the bass staff, is C4; A4, the tuning-fork A, vibrates at 440 Hz, and the piano runs from A0 to C8.
Careful: some solfège methods use the Franco-Belgian system, in which middle C is Do3. This site always uses scientific pitch notation, the international one: C4.
Not everyone numbers the same way
This site calls middle C C4, like scientific pitch notation, the most widespread today. But there are others, worth knowing so as not to be confused by a book or a music program.