An orchestra ( or ; ) is a large instrumental ensemble used in classical music that contains sections of string (violin, viola, cello and double bass), brass, woodwind, and percussion instruments. Other instruments such as the piano and celesta may sometimes be grouped into a fifth section such as a keyboard section or may stand alone, as may the concert harp and, for 20th and 21st century compositions, electric and electronic instruments. The term orchestra derives from the Greek ὀρχήστρα, the name for the area in front of an ancient Greek stage reserved for the Greek chorus. The orchestra grew by accretion throughout the 18th and 19th centuries, but changed very little in composition during the course of the 20th century.