An étude (French for 'study') is a solo instrumental piece. Originally études were conceived as technical exercises, focusing on a particular technique and structured to work as practice material for perfecting a particular skill. During the 19th century, the growing popularity of the piano as a home instrument helped develop a tradition of etude-writing, that culminated with études becoming concert pieces. Nowadays, certain sets of études are still used in a pedagogical context (such as those by Sor, or Clementi), while études by composers such as Chopin, Liszt, or Debussy, have acquired a repertoire status, and proven to be among the most difficult pieces in the repertoire.