In modern international acoustic and voice pedagogy usage, pitch designations are indicated by an Acoustical Society of America system that is endorsed by the U.S.A. Standards Association. Each keyboard octave, beginning at the note C, is assigned a number. For example, middle C (C4) initiates the fourth octave.
When performance literature for the male voice is notated in the treble clef, the indicated pitch is sung an octave lower than the same note designated for the female voice. (For example, from Puccini's La bohème act 1 duet, Mimi's High C occurs at C6 and Rodolfo's at C5).
Older treatises retain the Helmholtz method, often using upper and lowercase designations extending from C1 to c4. The Helmholtz method has universally been replaced by the more current U.S.A. Standards. A comparison of the two systems follows.