To a greater extent than most other instruments an ensembles, it is difficult to compose music for the guitar without either proficiency in the instrument or close collaboration with a guitarist. As a result, a large part of the guitar repertoire consists of works by guitarists who did not compose extensively for other instruments. Music prior to the classical era was often composed for performance on various combinations of instruments, and could be adapted by the performer to keyboard instruments, the lute, or the guitar. Since the beginning of the 20th century, however, a significant amount of music has been written for the guitar by non-guitarist composers.