Orchestras have appeared in many forms throughout the centuries, dating back as early as ancient Egypt. Orchestras that more closely resemble contemporary ensembles were formed in the 16th century, but with different types of instrumental placement. The instruments that are found in orchestras today began to appear in the 17th century. Around the 18th century, orchestras began to include brass and woodwind sections, and some percussion.
In an orchestra, the first violins are always seated at the left of the conductor, and the percussion instruments are placed behind the rest of the instrumentalists. The concertmaster, or the first chair violinist, sits on the outside of the first row to the left of the conductor and is responsible for the orchestra's tuning. In addition, each section of instruments includes a principal instrumentalist.
A standard orchestra placement follows a set of guidelines. Generally, the violin section is placed to the left of the conductor, and the viola section, cello section and string bass section are to the right. The woodwind section is often placed in the middle of the high and low string sections and usually includes clarinets, bassoons, flutes and oboes. Placement of woodwind instruments varies. Often the brass section--which includes horns, tubas, trumpets, and trombones--is placed behind the woodwind section. If there is a harpist, pianist or organist, she will often be placed in the area that is open or that lends itself to the type of music.
A standard orchestra is usually made up of 70 to 90 instrumentalists but may vary in size based on the type of music being played. In some instances, for example, the brass section is left out when the music requires only strings, woodwinds and percussion.
Based on the type of music being played, the size of the performance space or particular acoustic setup, the placement of instruments in an orchestra may vary. The entire setup generally follows the standard guidelines. However, the placement is entirely dependent on the preferences of the conductor, who may move instruments based on personal preference and/or sound quality.