What is col legno?
Col legno is an Italian term meaning “with the wood.” It is short for col legno battuto, “struck with the wood.” Traditionally, musicians play col legno by tilting their bow forward or backward and using a combination of hair and stick with a striking motion to get this percussive sound.
Where can I hear this?
A famous example of this is found at the beginning of “Mars” from Gustav Holst’s The Planets, where are all of the strings play col legno, adding to the rhythmic effect of the percussion section. In the following video, you can watch the strings of the BBC Symphony Orchestra playing this passage. Watch for the string basses and cellos at 1:10, and listen for the rhythmic clicking they are making.
Another good example of this technique is found in the first movement of Gustav Mahler’s Symphony No. 2 (“Resurrection”). In his notation, Mahler uses mit dem Bogen geschlagen (“struck with the bow”), the German equivalent of the Italian phrase. All of the strings use this technique for a couple of measures, and gradually go back to normal bowing ("gestrichen," or "stroked").
Watch Claudio Abbado conducting the Lucerne Festival Orchestra in this 2003 recording. You can see the violins playing this col legno passage at 15:06.
What does col legno mean for the safety of my bow?
Some musicians are very careful with their best wooden bows and keep a back-up bow when they know they’ll have to use the col legno technique. The stroke itself shouldn’t harm your wooden bow, but if you are just learning the technique and practicing a particular rhythm with it over and over, you may want to use a carbon fiber bow or Glasser bow until you are very comfortable with it. You probably don't need to buy a brand new bow just for one piece of music. However, if you are going to be doing a lot of symphonic playing and you are worried about breaking your expensive wooden bow, err on the cautious side and consider buying a sturdier bow for col legno playing.