Serialism
In 1923 the composer Arnold Schoenberg (1874-1951) developed what he called the “twelve-tone system”.  Instead of using just five notes as in our example, he used all 12 notes in the octave (5 black and 7 white notes on the piano keyboard).  Most music we listen to is in a particular key (tonality).  This is called “tonal” music.  For example: if we start “Twinkle, twinkle little star” on a C we finish on a C at the end and the piece sounds finished. It is “tonal” (in a particular key).  In twelve-tone music all 12 notes are equal, there is no “key note” (it is “atonal”).  This makes the music very hard to understand and it took people a long time to get used to the sound of Schoenberg’s new music.