The modern orchestra is a musical instrument upon which a performer, known as a conductor, plays compositions written especially for it. It is true that an orchestra is a collection of instruments, but these are intended to be so distributed and operated that the result shall be homogeneous, the effect that of one grand organ of sound. Within itself the orchestra embraces a wide variety of tone-qualities and many grades of power and brilliancy, and these are due to the presence of several different families of instruments, each having general qualities, with special traits in the individuals. It is by causing these different families to work together or separately that the composer achieves the expression of his thought, and it is by governing wisely the operations of the individual members and the families that the conductor conveys the composer's design to the hearer.