Quartet, in music, a composition for four voices, or for four instruments, in which the parts are all obbligati; that is, in which no one of the parts can be omitted without injuring the proper effect of the composition. To understand the structure of instrumental quartettos of this kind, those of Haydn, Mozart, Beethoven, Spohr, Onslow, and a very few others, must be examined. Vocal quartettos are generally accompanied by instruments, to sustain the voices. A mere interchange of melody between one part and another, where the parts alternately become principal and subordinate, without any interweaving of them by means of imitations, double counterpoint, &c. does not constitute a regular quartetto. Neither is a regular quartetto found in a composition that consists of one principal part, accompanied from beginning to end by three other parts, which are merely harmonic supports to the principal part. See Music and Principal.