some effect on intonation but some amount of intonation variability may be uncontrollable. Most electric fretted string instruments have individually adjustable bridge saddles, adjustable with a screw driver or Allen wrench. Acoustic fretted instruments typically have either a floating bridge, held in place by string tension, or a fixed bridge, such as a pin bridge on an acoustic guitar. A luthier or technician adjusts a floating bridge simply by carefully changing its position until the intonation is correct. Adjusting intonation on a fixed bridge involves carefully shaping the bridge saddle with a file to alter the string's contact point. Another cause of poor intonation on a fretted instrument is that the maker didn't cut the string slots in the nut deep enough. If the string is higher than fret height at the nut, the string deflection-caused pitch increase is progressively greater closer to the nut. Other instruments Like unfretted string instruments, the trombone relies on the musician precisely positioning something, in this case the trombone's slide. The slide's pitch adjustment on a single partial is approximately the interval of a tritone on a slide length of over 80 centimeters. The trombonist may use his/her ear to minutely adjust pitch on sustained notes. By coincidence, the position of the bell on most trombones provides a reference for fourth position, but this is not accessible during fast passages and is massively unreliable as intonation varies by instrument make and model. Articulation is also an important consideration when playing trombone: imprecise articulation may add unwanted glissandi to a note not requiring it. Imprecise articulation often ends up as poor intonation and tone. Woodwinds are manufactured with holes that must be covered or uncovered to shorten or lengthen the bore to change pitch, and (except for flute) require register keys to change octaves. Most woodwind instruments comply to a regular harmonic series of pitches and require one register key to change octaves. The exception is the clarinet, which sounds only every other pitch of the harmonic series. This means that the clarinet, rather than having an octave key, has a "register key" that raises the pitch by a 12th. Brass instruments with valves have an inherent intonation defect in that valve combinations tend to be sharp. This is because the open horn is in a pitch of (for example) B, so depressing the first valve lowers the pitch to A. Depressing the second valve