“Col Legno” on Strings
A quite interesting sounding and more commonly used special playing technique on string instruments is the so called “col legno”, which translated means “with the wood”. Probably one of the most well known use of it is the beginning of Holst’s Mars. Those notes are executed by turning the bow around and striking the strings with the wood part of it.
However in practical use string players usually don’t turn the bow around by 180° but just by 90° using a bit of hair as well, as you wouldn’t be getting any tone but just a click noise out of the string if you just used the wood part only. Some string players will also be concerned damaging their (sometimes massively expensive) bow by executing this technique too heavily so they might actually switch to a cheap bow, a pencil or even a metal nail for col legno passages.
Another important additional thing to consider is the fact that the bow bounces differently off the string when playing col legno than it does when playing with the hair which sometimes takes string players a while or few takes to get figures with more busy rhythms tightly together, so prepare for a bit of extra time on extensive col legno passages.
Also, be aware that even when executed fortissimo, this effect is quite soft. The example from Mars above uses the whole string section but still would be covered quite easily if not orchestrated accordingly.
Lastly, the sound is more characteristic on lower strings in lower registers of the instrument. Particularly high violin col legno passages tend to become more of a click sound rather than a tone.