Minimalism began as a general reaction to post-WWII modernism, atonality, and integral serialism. As such, there are certain traits that one find in minimalist pieces, but the technique is not uniform (just as there are many ways of composing 12-tone music, or classical period tonal music, for example).
Characteristics of minimalism:
- restricted pitch and rhythmic materials
- diatonic/modal pitch content
- pitch centricity
- use of repetition
- short rhythmic cells/patterns
- steady pulse
- the use of process
- often long duration (for works)
Other characteristics appear more in a particular composer.
Phasing is a technique most often used by Steve Reich. Reich will repeat a pattern in multiple instruments, then shift the relationship between the instruments to create changing cross-patterns of accents. In more recent works Reich focuses on short-time canons, which he referes to as “canons at the unison.”
Terry Riley uses drones, ostinatos, and indeterminacy in his minimalist compositions. As Riley is influenced by free jazz, his use of indeterminacy in performance can be seen as relating to jazz.
Philip Glass early works use short rhythmic patterns that are subject to new groupings to change the speed of the beat.
La Monte Young often focuses on a single drone that last for vary long durations with subtle variations.
It should be noted that not all minimalist pieces are tonal/diatonic. Some do not even use pitched material. Diatonicism and tonality are just much more commonly used in minimalism than atonality.
Other composers that are associated with minimalism include John Adams (Grand Pianola Music, Short Ride in a Fast Machine), Arvo Pärt (Fratres, Tabula Rasa), Frederic Rzewski (Coming Together, Attica), and Brian Eno (Music for Airports).
Post-Minimalism obviously draws upon many of the same traits of minimalism, often exhibiting more overt rock influences (rhythm, instrumentation, the prominence of beat patterns).