Balanced though it may seem from the outside, with two violins, two violas, and two cellos, the string sextet can become dangerously unbalanced. The most organic balance for stringed instruments, seen both in string quartets and symphony orchestras, is to have two violin voices, and one each for violas and cellos. String quintets, with notable exceptions, usually are written for two violas, and can ride on soloistic roles for certain players. But in the wrong compositional hands, a string sextet can sound weirdly thick. Two cellos is too often too many.
But Johannes Brahms wrote two fantastic string sextets. Why they work as well as they do is hard to describe, but it certainly arises from Brahms’ ability to keep his architecture evident (a bit like the George Washington bridge in New York — all the more beautiful and light for its exposed beams). His tunes, light though they may seem, are never merely tunes, but always shadows, mirrors, permutations and extensions of one another. There is not only drama, but also a kind of weight-sharing in the interplay of voices. So there is a kind of symphonic lightness in both of the sextets — especially the B-flat sextet, which bears a close resemblance to the serenades he wrote for orchestra — and that makes them enormously interesting to play. Everyone has structural elements; everyone lifts, every- one launches the piece toward friendly enormity.