What is...?
What is the pitch-class set analyser?
The pitch-class set analyser is a web tool for assistance in analysing and comparing segments of atonal music using (as you might have guessed) "pitch-class set analysis", whose chief proponent is Allen Forte.
What is, then, pitch-class set analysis?
Pitch-class (pc) set analysis is a method for comparing the pitch-class contents of (segments of) pieces of atonal music to each other and, according to its proponents, making explicit (some) structural properties of such pieces.
The fundamental unit of pc set analysis, making up the basis for various set theoretical comparisons, is the pitch-class ('pc') set -- an unordered set of pitches, disregarding octave, enharmonic spelling, repetitions, rhythmic values etc. The methods and the terminology are generally borrowed from mathematical set theory.
The concepts of pitch-class and pitch-class set -- as well as several others of relevance to pc set analysis -- were introduced in 1960 by Milton Babbitt (see for instance "Twelve-tone invariants as compositional determinants", Musical Quarterly 46:2, 1960, or "Set structure as a compositional determinant," Journal of Music Theory 5:1, 1961).
The hitherto most elaborate theory on pc set analysis, however, is proposed by Allen Forte, the standard work being his The Structure of Atonal Music (Yale University Press, 1973). Forte claims that his theory 'establishes a framework for the description, interpretation, and explanation of any atonal composition'.
Forte's book is the obvious choice if you have time for a 200-page account. In case you prefer some 20 pages, half a chapter of Nicholas Cook's A Guide to Musical Analysis (Oxford University Press, 1987) is devoted to Forte's system. If you rather want something like two pages, you may find the introductory text on this web site helpful.
The pitch-class set analysis is controversial. Its most notable critic is George Perle; see in particular "Pitch-Class Set Analysis: An Evaluation", The Journal of Musicology , Vol 8 (1990), 151-72. See also Author's declaration.