(Fr. quatuor; Ger. Quartett; Ital. quartetto), a composition for four instruments or voices. String Quartet.-The leading instrumental quartet is th a t written for two violins, viola and violoncello, and down to the close of the 19th century i t conformed in the main to the structure of the symphony or sonata. Indeed the development of the sonata form can be traced quite clearly in the string quartet. At first it concerned itself chiefly with melody played by the first violin and accompanied by the other instruments, b u t composers were not long in seeing the possibilities opened out by the establishment of a combination which possessed in its four members a similarity in tone-quality and an equally distributed flexibility in musical expression. Thus Haydn's quartets, even those written a t Weinzirl known as op. 1, whether originally intended for quartet or string orchestra, show an increasing perception of how to distribute the interest of the writing and make each instrument in turn take a pa r t of equal importance. Mozart's greater skill and feeling for polyphonic writing carried matters much further, while in the work of Beethoven the qua rte t reached the climax of the purely classical style. The ' romantic ' period left its mark upon the qua rte t in the evident desire for increasing the richness of the tone, and we find in the quartets of Schubert, Schumann, Mendelssohn and Brahms a ttempts to replace pure polyphony by massed tonal effects, the frequent use of double-stopping and the tremolo; in fact the character of independent part-writing sometimes disappears, and the writing approaches th a t of the orchestra both in point of view and sound. Still more has the orchestral influence been noticeable in later developments where a far more extensive use of the various ' colour ' effects of which the instruments are capable has been made. Eventually also other forms than the strict sonata form were taken up, as had already been done with the symphony and the orchestra, and we thus get quartets in one movement of the ' F antasy ' (q.v.) type, or short ' character ' movements described as ' pictures,' ' sketches ' and the like. The main classical repertory is as follows : Haydn, 83, of which the very early examples were possibly intended for string orchestra ; Mozart, 26; Beethoven, 16, and Fugue, op. 133, originally intended as finale to op. 130; Schubert, 3 ; Schumann, 3 ; Mendelssohn, 6 ; Brahms, 3 ; Dvorak, 8 ; Tchaikovsky, 3 ; Glazounov, 6 ; Borodin, 2 ; Reger, 4 ; Stanford, 5 ; S. Taneiev, 5 ; Smetana, 2 ; Verdi, Franck, Grieg, Wolf and Debussy each left one. More recent quartets are those of Bartok, Bax, Bloch, Bridge, Busoni, Elgar, Gibbs, Hindemith, Holbrooke, Honegger, Jarnach, McEwen, Milhaud, Novak, Pfitzner, Pizzetti, Ravel, Respighi, Schonberg, Scott, Smyth, Suk, Taillefere, Tovey, Turina, Vaughan Williams, Gerrard Williams and Charles Wood. P ianoforte Quartet.-Next in importance to the qua rte t for strings alone is th a t for pianoforte and strings (violin, viola and violoncello). Having its origin in the accompanied sonatas for one or more stringed instruments, the combination was found with the development of the piano and of the technique of piano playing to possess valuable qualities for musical expression. Obviously the effects are of a different nature from those of the strings alone, and while there must always be a disadvantage in the association of a fixed-tone instrument with those whose tones are free, this is compensated for by the difference in quality and the harmonic support which the pianoforte supplies. There are two pianoforte quartets of Mozart, four of Beethoven, three of which were early works and bear no opus-number, and Mendelssohn's opp. 1, 2 and 3, the combination practically coming into prominence with Schumann's op. 47 in E flat and the opp. 25, 26 and 60 of Brahms. There are also examples by Dvorak, 2 ; Faure, 2 ; Holbrooke, Novak, Parry, Reger, Saint-Saens, Stanford. Other quartet combinations are of course possible, such as th a t of Mozart for oboe, violin, viola and violoncello. (See Ch a m b e r M usic.) V o ca l Q u a r t e t s are so called whether accompanied or not. Both types have been greatly exploited in oratorio, alone and in combination with chorus, while the ensemble of a quartet of protagonists in opera was a regular feature of the older schools wherein the composer was often called on to show his skill in portraying simultaneously four different lines of thought or types of characterisation. The self-existing vocal quartet, soprano, alto, tenor and bass, with or without accompaniment, has, if one excepts the madrigal, a curiously small repertory. In point of fact, many madrigals and partsongs are equally suitable for a small choir. Otherwise outstanding works of this class are the two sets of ' Liebeslieder-Walzer,' opp. 52 and 65, the 1 Gypsy Songs,' op. 103, and quartets, opp. 64, 92 and 112, of Brahms, Schumann's ' Spanisches Liederspiel,' Henschel's ' Serbisches Liederspiel,' Faure's ' Pavane' and ' Madrigal ' ; and from English writers there may be mentioned 1 Six Pastorals ' and ' Nursery Rhymes ' (two sets) of H. Walford Davies, ' Songs of the River ' of T. F. Dunhill, songs from ' The Princess ' of Stanford and five songs from 'England's Helicon' of Ernest Walker. N. c . o.