"Wolfgang is well and is whiling away the time writing a quartet," Leopold Mozart wrote home from dreary Bolzano late in 1772, en route to Milan for the performance of his son's opera, Lucio Silla. The quartet alluded to in Leopold's letter was very likely one from the K. 155-160 group. Wolfgang A. Mozart (1756-1791) had composed a previous series of quartets (K. 136-138) in preparation for requests for chamber or sinfonia performances that might come up during his stay in Italy. Now, during the journey itself, Mozart broadened the repertoire with six more works.

The new quartets were outwardly written in the Italian taste - cast mostly in three-movement sinfonia form with light, lyrical slow movements and frothy, exuberant allegros. However, Mozart had made great strides in his craft and maturity since the earlier quartets and had added a new dimension. The second violin and viola parts had become much more important, especially in passages of contrapuntal development.

The Quartet in G is one of a group of early mature works by Mozart that can no longer be considered mere divertimentos for strings. They are already in a genuine chamber idiom and anticipate much that will follow in the "ten famous quartets" of his later years. As Alfred Einstein puts it, "Foreshadowings of the 'great' string quartets are found everywhere in these quartets by the 17-year-old Mozart; nor are they mere foreshadowings -- is spring only a foreshadowing of summer?" 

Dmitri Shostakovich (1906-1975) was for a very long time associated with the Beethoven Quartet, a Leningrad ensemble that premiered 13 of the composer's 16 quartets. The third and fifth quartets were dedicated to the group, and Shostakovich dedicated his Quartets Nos. 11-14 to its individual members. Thus, Quartet No. 13 featured the viola and was dedicated to the original violist of the ensemble (although by 1970, when Shostakovich completed the work, he was no long playing regularly as a member of the group). The Thirteenth Quartet is unique not only for moving the viola to the forefront, but it is also Shostakovich's only quartet composed in a single movement. Alan George, violist with the Fitzwilliam Quartet, writes:

The Thirteenth Quartet is indeed a harrowing experience for all involved; many listeners have been truly frightened by it, and even the most resilient emotional temperament could hardly fail to be at least uncomfortably disturbed by it.

One reason for this is the sheer intensity of certain moments in the work. Another factor is Shostakovich's use of 12-tone rows, highly chromatic and intense in close intervals. However, unlike many other serial composers, Shostakovich did not use dodecaphony to generate the whole piece. Rather, he created his themes out of 12 tones and used several 12-tone rows to do this.

The work opens with a brooding viola solo, but soon the violins join in the lonely texture. Out of the depths comes the cello, which leads the others into a series of intensified exchanges in the upper registers. This quiets to a murmuring support of the viola announcing the impending ostinato of the central section. The extremely affecting central portion of the quartet is a bitter, dissonant scherzo built on repetitive patterns. Particularly remarkable is the players' percussive tapping of the bellies of their instruments with their bows. The transition to the final section focuses on trills as the ostinato rhythm fades. Again, the eloquent viola solo returns, and we realize that the last section is a varied reprise of the first section. This becomes a coda as the viola echoes earlier motives to a sparse tapping accompaniment. The work ends unusually on a high unison that just seems to disappear.

With the two quartets of Op. 51, Johannes Brahms (1833-1897) had made his entry into the critically difficult world of the string quartet. Those works of 1873 had been serious and tense, worthy of comparison to Beethoven's middle quartets. Now, in 1875, with the initiation behind him, Brahms could relax and compose a more "unbuttoned" style of chamber music. In many ways, the B-flat Quartet is the antithesis of the previous two. It has a lighter overall mood, a pastoral character, a flexible rhythmic approach, and even some moments of humor. On hearing the quartet read by Joseph Joachim's ensemble, Clara Schumann wrote to Brahms,

I am especially pleased with the third and fourth movements and cannot decide which delights me more, the melodious viola solo in the third or the charming theme with its delicate tracery in the fourth. The theme with its playful ending is pure joy.

The first movement engages the ear immediately with its horn call theme, a little reminiscent of Mozart's "Hunt" Quartet. After a bustling transition comes a change of meter from triple to duple and a new theme that has been dubbed a "Czech Dance." The development, involving both themes, is framed by a new and delicate sotto voce theme. Toward the end of the movement, Brahms cleverly combines duple and triple rhythmic patterns in some original ways.

The expressive Andante is a singing three-part movement. Symmetry in the phrases of the outer sections is counterbalanced by searching modulations and rhythmic shifts in the central portion.

In place of a scherzo, Brahms offers as a third movement an unusual Agitate. The main section features the Cinderella instrument -- the viola -- in a warm and rich melody. (The violins and cello are muted , but the viola is not.) The Trio section blends the four instruments, and the music becomes gradually intensified before returning to the more placid main section.

Biographer Karl Geiringer considers the fourth movement to be the quartet's center of gravity. Its poised, quasi-Classic theme introduces a series of imaginative variations. The last of these reverts to the triple rhythms and the actual opening theme of the first movement, blending it into the flow. This, Geiringer points out, " ... leads the joyous movement to its climax."