The harpsichord was replaced as the main keyboard instrument by the piano (or fortepiano). Unlike the harpsichord, which plucks strings with quills, pianos strike the strings with leather-covered hammers when the keys are pressed, which enables the performer to play louder or softer (hence the original name "fortepiano," literally "loud soft") and play with more expression; in contrast, the force with which a performer plays the harpsichord keys does not change the sound. Instrumental music was considered important by Classical period composers. The main kinds of instrumental music were the sonata, trio, string quartet, quintet, symphony (performed by an orchestra) and the solo concerto, which featured a virtuoso solo performer playing a solo work for violin, piano, flute, or another instrument, accompanied by an orchestra. Vocal music, such as songs for a singer and piano (notably the work of Schubert), choral works, and opera (a staged dramatic work for singers and orchestra) were also important during this period.