It's a small mechanical organ with a wooden cylinder, the barrel, in which pins are set. The barrel is turned by a handle and the pins open the pallets to admit the air to the pipes. The barrel organ was used widely in churches in the 18th and 19th centuries when an organist was not available. Surviving barrels are important documentary evidence of the extent to which music was ornamented in performance, since the ornaments, which must be set on the barrels, were not included in written music. Small barrel organs were also used by itinerant street musicians and, by inaccurate transference, the name was also applied to the later street piano which was also played by a barrel.