Serialism is the important 20th century compositional technique that uses, as a basis of unity, a series of the twelve semitones of the octave in a certain order, which may then be taken in retrograde form, in inversion and in retrograde inversion, and also in transposition. The technique, an extension of late romantic chromaticism, was formulated by Arnold Schoenberg in the 1920s followed by his pupils Alban Berg and Anton Webern, and thereafter by many other composers. Problems arise for the listener in the difficulty of hearing the series, however visually apparent from the written score.