Most modern symphony orchestras have about a hundred performers who play from fifteen to twenty different instruments.  The combinations of sound possible are vast in number and effect.

Orchestration, the art of writing instrumental music to achieve a variety of effects, developed over the eighteenth century as the size of the orchestra grew.  In the nineteenth century, the orchestra expanded to include percussion instruments as well as more wind and string players.  The modern symphony orchestra includes strings, woodwinds, brasses, and percussion.

Usually, the strings and woodwinds play the melodic parts and can also be used to preserve harmony.  The brass instruments are usually given solo melodic parts, and are also used in music of a military nature.  The percussion functions mainly as a source of rhythm.  It is also used to create special moods and effects.