Like Josh said, the notation of C1 or C2, applies on piano.
Actually on piano, it begins with a A note so the C1 is the third tone on piano and then you have the C2, C3, C4, C5, C6 and C7, and it's the same thing for the others notes (D, E, G, A, B)
The C1 is the most low tone but I am...