Intonation is one of those things players often notice without quite being able to name: chords sound fine low on the neck, but something feels slightly off higher up. Here's what intonation actually means, and why it sometimes goes wrong.

What intonation means

Intonation is how accurately a guitar plays in tune with itself as you move up the neck. A guitar can be perfectly tuned when you play the open strings and still sound slightly sharp or flat on fretted notes further up, if the intonation isn't set correctly. It's checked by comparing the open string to the same string fretted at the 12th fret, which should be exactly one octave higher, with no drift in pitch.

What causes poor intonation

• Saddle position. The saddle needs to sit at a very slightly compensated position rather than exactly half the scale length, to account for the fact that pressing a string down to fret it very slightly increases its tension and sharpens the note. If the saddle isn't positioned correctly, intonation drifts as you move up the neck.
• String age and condition. Old, worn or dirty strings don't vibrate as consistently as fresh ones, and can genuinely throw off intonation even if nothing mechanical has changed.
• Action that's too high. Higher action means you have to press strings down further to fret a note, which increases how sharp fretted notes become relative to open strings.
• Neck relief issues. Similar to action, incorrect relief changes how far you're pressing strings down at different points on the neck, which affects intonation unevenly along its length.

What can and can't be adjusted

On most acoustic guitars, the saddle is a single fixed piece rather than individually adjustable per string, unlike many electric guitars. That means acoustic intonation correction is a more specialised job, sometimes involving reshaping or replacing the saddle, or in some cases adding compensation for a specific problem string.

When to get it checked

If chords sound fine but something feels wrong once you're playing higher up the neck, or a specific string sounds consistently off no matter how carefully you tune it, intonation is worth having checked. It's a routine part of a proper setup, and correcting it makes a genuinely noticeable difference to how in-tune a guitar feels across the whole fretboard.