Timbre is the unique quality of a sound, sometimes referred to as its tone color. It is determined primarily by the object that produces the sound. Composers can specify a specific timbre by indicating the instrument(s) for which they are writing.
Music for a melody instrument such as a violin or a trumpet is written on one staff. Music written with two or more staves are called scores. Keyboard instruments require two staves–one for the right hand and one for the left hand. This would be called a piano score (or as previously discussed, a grand staff).
Piano Score (excerpt)
A composition for an instrumental trio, for example, would use three staves, all joined by a brace, indicating that all the parts are played simultaneously. The following example is a score for three instruments: two violins and a viola.
String trio score