<P> The term orchestra derives from the Greek ὀορχήστρα (orchestra), the name for the area in front of a stage in ancient Greek theatre reserved for the Greek chorus . </P> <P> 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 . The term chamber orchestra (and sometimes concert orchestra) usually refers to smaller - sized ensembles of about fifty musicians or fewer . 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 . 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 . In recent decades some orchestras have returned to the Baroque, Classical and Early Romantic seating of musicians; second violins to the right, cellos and violas to the center which gives the music a stereophonic effect . </P> <P> 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 . 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 . </P> <P> 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 . 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 theater (e.g., Gilbert and Sullivan operettas). </P>

Who is known as the leader of an orchestra