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A R T I C L E S
Capos and intonation
by Bruce Morrison
Tuning an instrument with a capo can be frustrating. Tuning a capoed instrument to an uncapoed instrument or tuning 2 guitars to each other which have capos at different frets can try your patience. Why won't some instruments play in tune when capoed?
Poor intonation is the result of incorrect saddle placement, inaccurate saddle compensation, high string action, excess capo tension, or a combination of those factors. Determined by saddle placement and compensation, accurate string length is critical to proper intonation. When a string is too short, fret placement is in effect too high relative to the shorter string length. Though the open string is in tune, it notes out progressively sharper at every fret. When a string is too long, the reverse is true.
Fretting stretches a string increasing its tension which raises its pitch. Although instruments are built to compensate for some stretching, with high string action fretting stretches a string more than an instrument is designed to take into account. No incorrectly compensated string can play in tune. When the string action is too high a change of 1/32nd" in string length can make an audible difference to its intonation.
Guitars with high string action capoed up the neck readily exhibit poor intonation. Since a capo exerts more downward force than your fingers, it stretches strings more than fretting does. The more force a capo applies, the sharper the strings play. As it moves up the neck string action rises. So, the higher a capo is placed, the more stretching occurs producing further sharping.
The most effective capos have 2 features in common. One is the ability to control how much tension the capo applies. The minimum amount of tension to get a good clean sound is all that's needed. Any additional capo tension produces only unnecessary sharping as it needlessly presses the strings to the fretboard. Using less capo tension significantly reduces sharping, fret wear, and the likelihood of damaging the finish of your guitar's neck. The other feature is a capo pad made from a less resilient material. Since string windings make shallower impressions in a harder pad, strings move back and forth beneath it more smoothly. Less drag and tension makes tuning with a capo on much easier.
One school of thought says to place a capo right next to the fret, the idea being that sharping is reduced because the strings are not stretched as much. While I agree in principle, in real life a compromise is necessary. Because of the angle at which the hand wraps around the neck, a capo positioned parallel with and adjacent to the fret doesn't allow enough clearance for the index finger. Placing the bass side of the capo at the fret and angling the treble side 1/4 of the way back toward the lower fret keeps sharping at a minimum and provides adequate hand clearance.
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