Serialism
In the 1950s and 1960s some composers like Pierre Boulez (b.1925) and Karlheinz Stockhausen (b.1928) wrote music which was not only twelve-tone, but serial in other ways as well.  For example: they organized levels of dynamics (from very quiet to very loud) in a series, and the lengths of notes (from very short to very long), and even timbre (the sound quality).  By combining all these things their music became very complicated indeed.  When all these things were used for the whole piece of music it was called “total serialism”.  Boulez’s Structures I is an example of a piece which uses “total serialism”.  A lot of people criticized this music, saying that it was too mathematical instead of being expressive.  Such music can be hard to understand.