Intonation — In tune open, out of tune up the neck?

You tune the open strings, but chords further up the neck sound wrong? One possible cause is intonation: how accurate the notes are along the fretboard.

Adjusting it sets the effective length of each string, where the bridge allows it. The check usually compares the fretted note at the 12th fret with the harmonic at the same spot, without pressing the string too hard.

String height, the nut and string gauge all affect the result. That's why the check is done with strings in good condition and after the other adjustments are finished. If you change sets or tuning, it may need checking again. The goal is the best possible pitch balance across the whole fretboard.