Schoenberg realised that it was difficult to keep a piece going without the structure of keys and scales
He devised a Twelve-Tone Technique which he called a “Method of Composition with Twelve Tones which are Related Only with One Another”

Using serialism, all 12 semitone pitches are treated as equal, and the music is not based in a particular key or scale.

The composer creates a tone row using all 12 pitches in a chosen order – each pitch is used once only.