TWPitch facilitates the use of pitch class notation in SuperCollider. Pitch class notation is a system in which pitches and their enharmonic equivalents are identified by integers ranging from 0 to 11:

0:  C
1:  C sharp / D flat
2:  D
...
9:  A
10: A sharp / B flat
11: B

Octaves are identified by integers ranging from 0 to inf. Pitch class 0 in octave 0 produces C at ~8.176Hz. Pitch class 9 in octave 5 produces A at 440.0Hz.

Check out the source on Github.