An orchestra (/ˈɔrkɨstrə/ or US /ˈɔrˌkɛstrə/; Italian: [orˈkɛstra]) is a large instrumental ensemble 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 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.A smaller-sized orchestra for this time period (of about fifty musicians or fewer) is called a chamber orchestra. A full-size orchestra (about 70-100 musicians) may sometimes be called a symphony orchestra or philharmonic orchestra; these modifiers do not necessarily indicate any strict difference in either the instrumental constitution or role of the orchestra, but can be useful to distinguish different ensembles based in the same city (for instance, the London Symphony Orchestra and the London Philharmonic Orchestra). A symphony orchestra will usually have over eighty musicians on its roster, in some cases over a hundred, but the actual number of musicians employed in a particular performance may vary according to the work being played and the size of the venue. A leading chamber orchestra might employ as many as fifty musicians; some are much smaller than that. The term concert orchestra may sometimes be used (e.g., BBC Concert Orchestra; RTÉ Concert Orchestra)—no distinction is made on size of orchestra by use of this term, although their use is generally distinguished as for live concert. As such they are commonly chamber orchestras. There are several types of amateur orchestras, including school orchestras, youth orchestras and community orchestras.Orchestras are usually led by a conductor who directs the performance by way of visible gestures. The conductor unifies the orchestra, sets the tempo and shapes the sound of the ensemble.