Given the wide range of styles in classical music, from Medieval plainchant sung by monks to Classical and Romantic symphonies for orchestra from the 1700s and 1800s to avant-garde atonal compositions for solo piano from the 1900s, it is difficult to list characteristics that can be attributed to all works of that type. However, there are characteristics that classical music contains that few or no other genres of music contain, such as the use of a printed score and the performance of very complex instrumental works (e.g., the fugue). As well, although the symphony did not exist through the entire classical music period, from the mid-1700s to the 2000s the symphony ensemble—and the works written for it—have become a defining feature of classical music.

Answer this question, if possible (if impossible, reply "unanswerable"): Avant-garde atonal compositions were written for what instrument?
solo piano