[custom_adv] An orchestra is a large instrumental ensemble typical of classical music, which mixes instruments from different families, including bowed string instruments such as violin, viola, cello, and double bass, as well as brass, woodwinds, and percussion instruments, each grouped in sections. [custom_adv] Other instruments such as the piano and celesta may sometimes appear in a fifth keyboard section or may stand alone, as may the concert harp and, for performances of some modern compositions, electronic instruments. [custom_adv] A full-size orchestra may sometimes be called a symphony orchestra or philharmonic orchestra. The actual number of musicians employed in a given performance may vary from seventy to over one hundred musicians, depending on the work being played and the size of the venue. [custom_adv] Orchestras that specialize in the Baroque music of, for example, Johann Sebastian Bach and George Frideric Handel, or Classical repertoire, such as that of Haydn and Mozart, tend to be smaller than orchestras performing a Romantic music repertoire, such as the symphonies of Johannes Brahms. [custom_adv] the typical orchestra grew in size throughout the 18th and 19th centuries, reaching a peak with the large orchestras (of as many as 120 players) called for in the works of Richard Wagner, and later, Gustav Mahler. [custom_adv] Orchestras are usually led by a conductor who directs the performance with movements of the hands and arms, often made easier for the musicians to see by use of a conductor's baton. [custom_adv] The conductor unifies the orchestra, sets the tempo and shapes the sound of the ensemble. The conductor also prepares the orchestra by leading rehearsals before the public concert, in which the conductor provides instructions to the musicians on their interpretation of the music being performed. [custom_adv] The leader of the first violin section, commonly called the concertmaster, also plays an important role in leading the musicians. In the Baroque music era (1600–1750), orchestras were often led by the concertmaster or by a chord-playing musician performing the basso continuo parts on a harpsichord or pipe organ, a tradition that some 20th century and 21st century early music ensembles continue. [custom_adv] Orchestras play a wide range of repertoire, including symphonies, opera and ballet overtures, concertos for solo instruments, and as pit ensembles for operas, ballets, and some types of musical theatre (e.g., Gilbert and Sullivan operettas).
