Col legno
Using the wood backing of a bow to hit the strings of an instrument
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In music for bowed string instruments, col legno, or more precisely col legno battuto (Italian for 'with the wood [being hit]'; pronounced [kol ˈleɲɲo batˈtuːto]), is an instruction to strike the string with the stick of the bow across the strings.

20 sec: ... with tremolo,
33 sec: ... with glissando,
48 sec: battuto with movement of the bow across the fingerboard
History
The earliest known use of col legno in Western music is to be found in a piece entitled "Harke, harke", from the First Part of Ayres (1605) by Tobias Hume. He instructs the gambist to "drum this with the backe of your bow".[1] Such 'drumming' or col legno battuto also occurs in Mozart's 'Turkish' Violin Concerto No. 5, Haydn's Symphony No. 67, Salieri's La Cifra, Chopin's Piano Concerto No. 2, and Berlioz's Symphonie fantastique.
Col legno tratto ("drawn") is rarer effect: Mahler felt the need to include a note "No mistake! to be stroked with the wood" in the third movement of his First Symphony[a]. Tratto occurs in many works of the Second Viennese School, such as Webern's Four Pieces for Violin and Piano, Op. 7, Berg's Lyric Suite[b] and Schoenberg's Moses und Aaron.
Sound
The percussive sound of battuto has a clear pitch element determined by the distance of the bow from the bridge at the point of contact. As a group of players will never strike the string in exactly the same place, the sound of a section of violins playing col legno battuto is dramatically different from the sound of a single violin doing so.
The wood of the bow can also be drawn across the string – a technique called col legno tratto ("with the wood drawn"). This is much less common, and the plain marking col legno is invariably interpreted to mean battuto rather than tratto. The sound produced by col legno tratto is very quiet, with an overlay of white noise, but the pitch of the stopped note can be clearly heard. If the sound is too quiet, the bow can be slightly rolled so that a few bow hairs touch the string as well, leading to a slightly less "airy" sound.