An orchestra is a large instrumental ensemble that contains sections of string, brass, woodwind, and percussion instruments. The term orchestra derives from the Greek ορχήστρα, 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 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 theLondon 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. Orchestras can also be found in schools. 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.
The typical symphony orchestra consists of four proportionate groups of similar musical instruments called the woodwinds,brass, percussion, and strings. 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 orchestra, depending on the size, contains almost all of the standard instruments in each group. In the history of the orchestra, its instrumentation has been expanded over time, often agreed to have been standardized by the classical period and Beethoven’s influence on the classical model.
The so-called « standard complement » of double winds and brass in the orchestra from the first half of the 19th century is generally attributed to the forces called for by Ludwig van Beethoven. The exceptions to this are his Symphony No. 4, Violin Concerto, and Piano Concerto No. 4, which each specify a single flute. The composer’s instrumentation almost always included paired flutes, oboes, clarinets, bassoons, horns and trumpets. Beethoven carefully calculated the expansion of this particular timbral « palette » in Symphonies 3, 5, 6, and 9 for an innovative effect. The third horn in the « Eroica » Symphony arrives to provide not only some harmonic flexibility, but also the effect of « choral » brass in the Trio. Piccolo, contrabassoon, and trombones add to the triumphal finale of his Symphony No. 5. A piccolo and a pair of trombones help deliver storm and sunshine in the Sixth. The Ninth asks for a second pair of horns, for reasons similar to the « Eroica » (four horns has since become standard); Beethoven’s use of piccolo, contrabassoon, trombones, and untuned percussion—plus chorus and vocal soloists—in his finale, are his earliest suggestion that the timbral boundaries of « symphony » might be expanded for good. But for several decades after his departure, symphonic instrumentation was faithful to Beethoven’s well-established model, with few exceptions.