I have had the good fortune to accompany many great songwriters. I have, on occasion encountered the phenomenon I like to call Capo versus tuning.
First, a little bit about tuning.
When a guitar is in tune, unless a string slips or stretches, playing it cannot make it go flat. However, strings can be inadvertently pulled sharp. For example, if a string is pulled to the side or pressed harder than necessary to reach the fret the extra tension will the make the note sharp. Along those same lines, if action is too high, or if the nut slots are too high, the strings will be need to be pulled farther than necessary resulting in sharp notes.
When one plays a barre chord and has two fingers on a given string it can happen that one squeezes a little too hard and you can hear the “out-of-tune-ness”.
Given the nature of a tempered fret scale (fret spacing), a major third already sounds a little sharp, there is no tolerance here for more “sharpness”. If one presses any harder than necessary to get a clear tone from a string it will be a little sharp. This becomes even more of a problem with on a guitar with larger frets. Similarly, lighter strings are pulled more easily out of tune. Now we have an understanding why notes on a guitar tend to sound sharp not flat.
A little bit about Capos
Capos serve many purposes. The main one is that it allows us to change key easily by shortening the length of the scale(neck). Like your fingers if a capo applies any more force than necessary it will pull the strings sharp. A capo is also likely to pull the low strings even more because they are thicker and sit higher above the frets. Placing a capo closer or farther to the fret will help one to account for these inconsistencies. I favor some capo designs over others. I like a clamp style capo that can be adjusted to just the right amount of tension. Some people prefer a spring loaded capo which may be easier to use, but this type of capo is always pressing as hard as it can, more likely to cause of “out-of-tune-ness”.
capo on guitar
A little bit about accompanying capo users.
When one is playing by oneself tuning problems are not so noticeable. If you are playing with someone else who is not using a capo. You are likely to find yourself sounding a little sharp which can make your accompanist sound bad.
Choosing the right capo and a little technique will work wonders. Any time one applies or moves a capo they should be prepared to a little fine tuning (after moving the capo !). An electronic tuner is the best effect pedal money can buy. Your capo need not be an enemy in the battle for good tuning.