20th century 'classical' music

Minimalism Electronic music:
Atonal music: A piece of music that is played by a load of random Notes, not in a specific key.

A musical example of Atonal music

Aleatoric music: music of chance, 1950's 60's
Serialism: using every single note in an octive, uses the whole of the chromatic scale. All the notes are it in any random order and you had to use all the notes and they could be used in an order. Schoenberg the inventor of serialism. Play it forwards and backwards (inverted) then you play it in retrograde, retrograde is where every interval between notes would be inverted.

a musical example and description of Serialism