Intonation is the adjustment that makes your guitar play in tune all the way up the neck, not just at the open strings. A guitar with bad intonation will be perfectly in tune on open strings and progressively sharper or flatter as you move toward the body. Here is what causes it and how to fix it.
Why intonation needs to be set
The physics: a guitar is tuned by dividing the string at the 12th fret into exactly half its vibrating length. The 12th fret should produce a note exactly one octave higher than the open string. If the vibrating length is slightly off, the 12th fret note is sharp or flat relative to the open string.
Factors that affect the required saddle position: string gauge, string height (action), string stiffness, fret height, and the nut position. Any time you change strings or adjust the action significantly, re-check the intonation.
What you need
A chromatic tuner. Phone tuner apps work for this. A clip-on tuner or a tuner pedal is more accurate if you have one.
The measurement
Tune the open string to pitch. Then fret the same string at the 12th fret and read the pitch without bending or squeezing. Compare the two:
- If the fretted 12th fret note is sharp (above pitch), the saddle needs to move back (away from the nut). This lengthens the vibrating string length.
- If the fretted 12th fret note is flat (below pitch), the saddle needs to move forward (toward the nut). This shortens the vibrating string length.
Using harmonics for verification
Touch the string lightly directly above the 12th fret wire (do not press down) and pluck. This produces the 12th fret harmonic. It should match the fretted 12th fret note. If the fretted note is sharp of the harmonic, move the saddle back. If flat, move forward.
How to adjust the saddle
On a tune-o-matic style bridge: each saddle has a small screw at the back that moves the saddle forward or back. On a fixed hardtail bridge with individual saddles: there is usually a similar screw accessible from the back. On vintage-style synchronized tremolo bridges: the saddle intonation screws are at the back of the bridge plate.
Adjust by small increments. Each adjustment requires you to retune before checking again, because moving the saddle affects the string length and therefore the pitch of the open string.
Why you need to retune between checks
After moving a saddle, the open string pitch changes. If you check the 12th fret without retuning, you are comparing a detuned open string to a detuned fretted note and the relationship is meaningless. Retune every time.
Common mistake: high action causing intonation problems
When you fret a string, you are stretching it slightly toward the fretboard. The higher the action, the more stretch and the sharper the fretted note. If your intonation is consistently sharp on all strings, check your action height. A high action can push saddles so far back that they run out of travel. Fix the action first, then set intonation.
Correct intonation is audible. Playing a chord up the neck and having it sound in tune is one of the most satisfying results of a thorough setup.