to pitches without specific octave designations (A3, C#5, etc.), serial
composers think in terms of pitch classes. Pitch class numbers
from 0 to 11 are used to refer to these pitch classes. The most common
labeling system regards the first note of a piece as pitch class 0. A
chromatic scale beginning on this pitch assigns each of the remaining
11 pitches to a specific pitch class number for that piece.
For example, if a composition begins with the pitch class D, the chromatic scale beginning on D assigns the remaining pitch class numbers as follows:
And, if a composition begins with the pitch class F-sharp, the chromatic scale beginning on F-sharp assigns the remaining pitch class numbers as follows:
In the notation of serial music, an accidental applies only to the pitch which immediately follows and not to subsequent occurences of that pitch within the measure - a departure from traditional notation.
observe that enharmonic spellings (Eb and D#, for example) are used
indiscriminately in serial music unlike in tonal music.