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The orchestra is a large instrumental ensemble composed by instrument of different families and subfamilies. It’s usually composed by string instruments, woodwinds, brass and percussion instruments, grouped in sections. Other instruments can play in the orchestra, such as the piano, the celesta, etc. but that’s unusual. The number of musicians depends on the instrumentation, which is the number of instruments which the composer chose when s/he composed the piece. The number of musicians which play in an orchestra fluctuate between 20 and 120 in the larger instrumentations of the XIX and XX centuries.
At the beginning, the firsts orchestras (Baroque period) were very small, they were small and the only woodwinds instruments which played were the Flute, the Oboe, the Bassoons and Trumpets or/and Horns occasionally. The only percussion instrument played was the timpani. In the classical period the clarinet were added to that list and the horns and trumpets finally played regularly. In the romantic period appeared the Trombones, the Tuba and the Harp. There were other instruments of the families which joined to the group, such as the Piccolo, the English Horn, the Contrabassoon, etc., the Percussion section grew, adding the Bass Drum, the Snare Drum, the Tam-Tam, etc. and other instruments also joined, like the Piano, the Celesta, etc.
The orchestras are usually led by a conductor who directs the performance moving his arms and using a baton. Her/his task is to unify the orchestra, set the tempo and look for an orchestral sound or timbre.
The first Violin is called Concertmaster, and s/he also has to lead the musicians. Regarding the wind section, the first Oboe is the leader as well as the Timpani in the Percussion section.