Béla Bartók’s Mikrokosmos (German for Microcosm) Sz. 107, BB 105 consists of 153 progressive piano pieces in six volumes written between 1926 and 1939.
All of the six volumes progress in difficulty, namely:
Pieces 1-36 and 37-66, beginner level
67-96 and 97-121, moderate to advanced level
122-139 and 140-153, professional level