Twelve Tones – an introduction
This video is a nice introduction to dodecaphonic music theory
Twelve-tone technique—also known as dodecaphony, twelve-tone serialism, and twelve-note composition—is a method of musical composition devised by Austrian composer Arnold Schoenberg (1874–1951). The technique is a means of ensuring that all 12 notes of the chromatic scale are sounded as often as one another in a piece of music while preventing the emphasis of any one note through the use of tone rows, an ordering of the 12 pitches. All 12 notes are thus given more or less equal importance, and the music avoids being in a key. The technique was influential on composers in the mid-20th century.
Schoenberg himself described the system as a “Method of Composing with Twelve Tones Which are Related Only with One Another”. However, the common English usage is to describe the method as a form of serialism.
Schoenberg’s countryman and contemporary Josef Matthias Hauer also developed a similar system using unordered hexachords or tropes—but with no connection to Schoenberg’s twelve-tone technique. Other composers have created systematic use of the chromatic scale, but Schoenberg’s method is considered to be historically and aesthetically most significant.