in the mid-20th century a group of composers known as the “darmstadt school” (because they often met in darmstadt) continued to write music which was based on serialism. this included pierre boulez and karlheinz stockhausen. many of them, including both boulez and stockhausen, also experimented with electronic music. the term avant garde is often used to describe their music. it means that it expands the limits, or pushes ahead into new ground (literally the “front guard”, a military analogy). other american composers were experimental, e.g. charles ives and john cage who is famous for using a “prepared piano” (a piano which makes strange sounds because strange objects are put inside it).