The neck of a classical guitar may look to be completely straight, but guitars which are well adjusted for playing have a degree of bend in the neck. You might notice that the playing action of the guitar changes if the neck bend becomes more or less pronounced. Neck bend can change over time, with changes in temperature or climate, or if you change gauges on your strings.
When the neck of a classical guitar is perfectly straight, the strings will typically buzz against the frets along the neck. This problem will be particularly noticeable when you are playing chords, or if the neck has a backwards bend -- known as a back bow -- which brings the frets closer to the strings. Most commonly, classical guitars are designed to have a very slight forward bend to keep the frets away from the strings and reduce unwanted buzzing.
Degree of Bend
The amount of forward bend that is best for a classical guitar will depend on the individual instrument, the type of strings you use and the playing style and personal preferences of the user. For a light, fast picking action, a gap of 0.004 to 0.006 inches between the strings and the guitar neck is common. With light strings, or heavy strumming, a gap of between 0.007 and 0.012 inches is more likely to avoid unwanted buzzing sounds.
You can measure the distance between your strings and your guitar neck using a capo and a feeler gauge. With your capo on the first fret of the guitar, press your finger on the high E string -- the thinnest string on the guitar -- at the point where the neck joins the guitar body. The string between your capo and your finger will be straight, showing how far the guitar neck bends away from the strings. Find the largest gap between the string and the neck -- likely around the halfway point between your capo and finger -- and measure this distance with the feeler gauge.
Truss Rod Adjustment
The neck of a classical guitar contains a truss rod which runs through the center of the neck from end to end. Adjusting the truss rod will change the amount of bend in the guitar neck. If the truss rod is adjusted as far as it can go and there is still no forward bend keeping the frets a sufficient distance from the strings, you can try changing your string gauge to affect a small degree of bend. Heavier strings will typically pull away from the guitar neck more than light strings.
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