The 19th century also saw a number of étude and study collections for instruments other than the piano. Guitarist composer Fernando Sor published his 12 Studies, op. 6 for guitar in London as early as 1815. These works all conform to the standard definition of 19th-century étude in that they are short compositions, each exploiting a single facet of technique. Collections of studies for flute were published during the second half of the 19th century by Ernesto Köhler, Wilhelm Popp and Adolf Terschak.