Here is an extract from a Gresham College lecture by Professor Roger Parker from 2007:
Although Haydn’s works are close to the start of the genre, so much so that he is sometimes even called its ‘father’, composers today are still writing them, often regarding them as compositions of great seriousness. Why has this particular combination of instruments lasted for so long? It’s not, after all, a particularly balanced group. Two violins, one viola (which is tuned a fifth lower), and one cello (which is an octave below that). We hear all sorts of quasi- mystical stuff about the famed ‘balance’ and ‘equality’ of the group, but in fact the differences between the instruments make the ensemble in some ways extremely problematic. A viola is bigger than a violin, which makes it louder, but also harder to play in tune, particularly when the playing is fast. And the cello is so much larger still that the distances the left hand has to traverse necessitate a radically different fingering system. All this means that music played on one instrument will not always transfer easily to another. To take only the most obvious example: a rapid melody that may be a walk in the park for the violins can become a steep mountain path for the viola; for the cello, an oxygen mask and advanced climbing gear may be needed.
This imbalance in the ensemble derives from its origins, which were in early eighteenth-century orchestral groups. The two violins there tended to function in what’s called a ‘trio-sonata’ texture, weaving in and out of each other’s line, frequently overlapping; and the viola and cello tended to supply little more than the functional bass part. No equality here (apart from in the two upper instruments). But if the string quartet started life as a kind ofminimal orchestra, for performances in smaller venues, many of them domestic, it soon took on a life of its own, becoming the most common chamber music ensemble of the later eighteenth century, first in Germany and Austria, then spreading, with the spread of its repertoire, to other European countries.
By the end of the eighteenth century, the string quartet’s prestige had become considerable, in large part because two of the most famous composers of the period, first Haydn and then Mozart, had dedicated some of their most complex music to the genre. Beethoven simply added to this prestige, and after him there was no looking back. Although the comparatively restricted and uniform sound of four solo strings might have seemed thin indeed for musicians of the nineteenth century, let alone for those of the twentieth, composers kept measuring themselves against the accumulation of masterpieces of the past. And so the string quartet has become a major repository for a certain kind of classical music; we might even say that it represents a particular attitude to what is central to our musical past. This is important, and not to be underestimated. The ‘rise’ of the string quartet more-or-less came with the rise of Austro-German instrumental music generally, and its prestige was locked into the idea that that particular tradition was central to our musical universe: was the one against which all others should be measured. This is why all those composers of the nineteenth and twentieth centuries – by no means all of them Austro-German – wanted to write string quartets, even at a time when many other combinations would have seemed more suited to the times. They wanted to measure themselves against the centre.