 Music OCR, software to "read" scanned sheet music so that the results can be manipulated, has been available since 1991.In 1998, virtual sheet music evolved further into what was to be termed digital sheet music, which for the first time allowed publishers to make copyright sheet music available for purchase online.Unlike their hard copy counterparts, these files allowed for manipulation such as instrument changes, transposition and MIDI (Musical Instrument Digital Interface) playback.