The bow can be drawn across the string at any point from just next to the bridge (ponticello) to up over the fingerboard (tasto). As with many of the techniques discussed here, players usually make their own decisions as to whether the more forceful and gritty tone achieved near the bridge or the more delicate sound created by moving towards the fingerboard is more appropriate. However, composers quite often specify the two extremes of as close to the bridge as possible (sul ponticello), which makes quite a harsh sound, or right over the fingerboard (sul tasto), which creates a whispery tone.
The fingered tremolo in the second violin and cello parts from the fourth bar of this extract from a Szymanowski quartet are played right over the fingerboard and therefore come out as a barely audible haze through which the main ideas can be heard:
Szymanowksi, Quartet No. 1, second movement
The melody in the Molto meno mosso is a striking example of a quiet sul pont. sound – harsh, glassy and without any of the warmth usually associated with string instruments.
Janacek, Intimate Letters, first movement
In this extract, the violins are sul tasto and the viola is sul pont. over a pizzicato cello line. The second violin moves to sul pont. after a few bars:
Symanowski Quartet No. 1, last movement
A related technique to sul tasto, and one that is nearly always performed towards the fingerboard, is to take nearly all the weight out of the bow in order to create a very floaty sound. The viola in this extract plays these short flurries with a quiet and ghostly tone.
Janacek, Intimate Letters, first movement (just before 14)