Guitar Intonation and String Gauges
A common frustration: every open string is in tune, but a chord played up the neck sounds sour. This is almost always intonation, meaning how accurately the guitar plays in tune at every fret, not only on the open strings. String gauge matters here too, because heavier strings change both tension and feel.
What intonation is
Fretting a string stretches it slightly and pushes its pitch up a little. Guitar makers compensate by placing the bridge saddle a few millimeters farther from the nut than the exact scale length, and by a different amount for each string. When that compensation is off, notes at higher frets drift sharp or flat even though the open strings are perfect.
How to check it
- Put on reasonably fresh strings. Old, worn strings never intonate well.
- Tune the open string precisely with the tuner.
- Play the harmonic at the 12th fret (touch the string above the fret wire without pressing it). It sounds exactly one octave above the open string and is not affected by the stretching described above.
- Now fret the same string at the 12th fret and play it normally. Compare it to the harmonic on the tuner.
If the fretted note is sharp, the string is effectively too short. If it is flat, the string is too long. A difference of a few cents is normal. More than about 5 to 10 cents is the point at which chords start to sound out of tune.
How to adjust it
- Fretted note sharp: make the string longer by moving the saddle back, away from the neck.
- Fretted note flat: make the string shorter by moving the saddle forward, toward the neck.
On most electric guitars each saddle has an adjustment screw, so this is a screwdriver job: move the saddle a little, retune the open string, check the 12th fret again, and repeat. Always retune after each change, because the adjustment alters the string’s tension. On an acoustic guitar the saddle is a fixed piece of bone or plastic, so changes are a job for a repair person. If the notes are sharp at higher frets on an acoustic, the cause is often action that is too high rather than the saddle position.
Intonation also depends on neck relief (the slight forward bow of the neck, set by the truss rod) and on the height of the strings above the frets (the action). High action means a longer stretch when you fret a note and therefore a sharper result, so correct those before moving saddles.
String gauges
Gauge is the thickness of a string, quoted in thousandths of an inch. Sets are named by the thinnest and thickest strings. Typical electric sets run from .009–.042 (extra light) through .010–.046 (the common regular set) to .011–.049 (medium). For acoustic guitars, light sets are around .012–.053 and medium sets around .013–.056.
A string’s tension at a given pitch depends on its mass per unit length and on its vibrating length: tension grows with the square of the frequency, the square of the length, and the string’s weight per length. In practice that means three things:
- Heavier strings, same pitch, more tension. They feel stiffer, sound fuller and are harder to bend. Lighter strings feel looser and are easier to bend, but can buzz if the action is low.
- Longer scale length, more tension. A 25.5 inch scale (typical of Fender-style guitars) feels tighter than a 24.75 inch scale (typical of Gibson-style guitars) with the same strings.
- Lower tuning, less tension. Tuning down with the same strings makes them floppy and likely to buzz. Players who tune down use heavier gauges to restore the tension: a set for half step down or drop tunings often runs one gauge heavier than the standard set, particularly on the low strings.
When you change gauge or tuning, expect to re-check the neck relief, the action and the intonation. They interact, and a set-up that was right for one string set is rarely exactly right for another. The guide to tuning a guitar covers getting the open strings right first.