Section 33.5 Forte Numbers
When Allen Forte created a catalog of every possible 3-, 4-, 5-, 6-, 7-, 8-, and 9-note set in Appendix 1 of The Structure of Atonal Music, he labeled each prime form with two numbers separated by a hyphen. His labels (3–1, 3–2, etc.) are now known as “Forte numbers,” and are seen in the tables in the Lists of Set Classes, which include prime forms and interval vectors as well.
Subsection 33.5.1 Z-Relations
“Z” in a set label (for example, 4–Z29) stands for “zygotic” (“twinned”), and is used when different prime forms (for example, 4–Z29 and 4–Z15 have the same interval vector—111111).