Why does my guitar sound out of tune higher up the neck?

Open strings can be perfectly in tune while fretted notes drift. As you go up the neck, small errors in saddle position add up.

How to check intonation:

1. Fit fresh strings and tune to pitch.
2. Play the 12th-fret harmonic on one string and check it on a tuner.
3. Fret the same string at the 12th fret, pressing normally, and check again.
4. If fretted is sharp, lengthen the string by moving the saddle away from the neck.
5. If fretted is flat, shorten it by moving the saddle toward the neck.
6. Retune and repeat until both readings match. Do each string.

Other causes to rule out:

• Old strings: worn or corroded strings intonate unevenly. Replace them first.
• Pressing too hard: heavy fretting pulls notes sharp, especially with high action.
• High action: the string stretches more when fretted.
• Gauge change: a new gauge usually needs new saddle positions.

On acoustic and classical guitars, the saddle is fixed, so intonation changes need a technician.