How to Inspect a Guitar Neck  

...also available in French at this location:  Manche de la Guitare

This discussion applies to classical, electric and acoustic steel string guitars. Although the classical nylon string guitar usually does not have an adjustable truss rod this method of evaluation is still applicable.

In my terminology, "warp" always means a neck coming forward as a consequence of the force exerted by string tension and usually produces high playing action. "Tilt" means a neck which is set back against the pull of the strings resulting in lower playing action.   Throughout this article I shall use the ideal neck set-up, which has a slight amount of warp (also called "relief"), as a constant point of reference.


To examine a neck first perform a preliminary visual inspection:

Grasping the fully strung guitar with both hands, bring the neck up to your line of sight (looking from the bridge toward the tuning keys) and aim it at a diffused light source, such as a bright window, so the reflected glare from this light illuminates the fingerboard. Concentrate only on the shadow of the strings on top of the frets, not the wood (also, don't look at the edge of the fretboard because playing wear can make it appear warped). With the upper frets over the body reading straight as a rifle barrel, the shadow will show a very slight dip around the 4th/5th/6th lower frets (on the ideal neck).

Take note if a fret (or group of frets) is obviously higher than its neighbors. This may indicate a fret rather than a neck problem. However, a group of protruding frets resulting in a visual high spot in the lower frets could be caused by a tilted or overadjusted neck.

Next, rotate the instrument end-for-end and repeat the procedure from the tuning machines point of view. Do the string shadows indicate a neck which is straight as an arrow; or, does the area of the fretboard over the body seem to deflect slightly up, down, or remain in line with the rest of the neck? Any abrupt or obvious deflections of the fretboard over the body indicate a limiting flaw in the overall playability of the guitar. Make note of your observations and recheck them.

(actually a three-part test conducted using the 6th and 1st strings with the goal of determining the extent of warp and the area of the neck where the warp is concentrated)

Now try and confirm your visual inspection results with a straightedge placed on the frets. The most convenient is the installed guitar string. Simultaneously press the 6th string at the 1st and 12th (or 14th for electric and steel string guitars) frets. Look carefully at the 6th there the tiniest gap or is the string touching all the frets? A 1/64" gap (.015" or .4 mm) indicates an acceptable warp for most guitars. If there is no gap the neck may be too straight (or actually tilting backwards) and the truss rod in need of loosening. But first you must check the rest of the fretboard.

Next, fret the string simultaneously at both the 8th and highest (19th, 20th, 21st, 22nd or 24th, depending on the type of guitar) fret. Is there a gap at the midpoint (12th to 14th frets)? If so, then there is warp over the body (the fretboard is rising) because this area should always read level (no gap).

Finally, fret the string at the 1st and last frets. If the 6th fret gap determined by the first straightedge test is unchanged then your warpage is concentrated in the part of the neck away from the body and is controllable by the truss rod adjustment. However, if the 6th fret gap increases then you also have some warpage over the body, a situation not likely remedied by truss rod adjustment alone. Keep the string down at the 1st fret (use a capo if necessary) and confirm your results by sliding down the fretboard starting from the highest fret, making contact with each fret as you slide. If the 6th fret gap decreases as you move then there is definitely fretboard deflection over the body.


If the entire neck is showing warp the visual inspection will reveal a continuous curve running the length of the fingerboard.  The ideal neck will only show warp concentrated in the neck area away from the body.  Remember, the upper area of the fingerboard should be dead straight and not show a gap when examined.

Use the straightedge test to cross-check and educate your eyes. With persistence, patience and practice you will be able to confirm the results. Check both sides of the fingerboard by repeating all three straightedge tests on the 1st and 6th strings: lower area, upper area, and the entire fingerboard with your finger pressing the string to the frets and sliding from the top down.


If we have the straightedge test then why bother with a detailed visual inspection? If the neck is perfectly straight or tilted back the straight string will give the same results because it will appear to touch all the frets. The visual inspection therefore separates the perfectly straight situation from the tilted (or over-adjusted) neck.  The visual inspection is also very important for detecting when the fingerboard drops over the body.

Rechecking with the straightedge test and also measuring action height changes at the 12th fret with a fine ruler will track the actual effect caused by small changes in the truss rod adjustment. Loosening (deadjusting) the rod will raise the overall height of the playing action and necessitate lowering both the string nut and bridge saddle height settings.

A six-inch metal ruler can find high frets when your visual inspection and straightedge tests indicate a straight neck. Fret misalignment (a high spot) of just a couple of thousandths will cause the ruler to "rock" on the protruding fret. Notes played just below this area might "buzz" or be cut-off completely by the high spot. If an individual fret is causing the problem it can be reseated or levelled. Also, a truss rod readjustment might level a high spot or, if the wood itself is distorted, the frets can be removed to allow planing of the board. Flip the ruler to the other edge and get the same results to confirm that the ruler itself is straight and true.


When you can reconcile and cross-confirm the visual observations with the straightedge tests there is enough information to determine if the neck is in need of adjustment or repair. Best of all, mastering this procedure can empower you when shopping for a new or used guitar. New guitars can be seriously flawed or just need a neck/fret adjustment.

Because the facets of a guitar's playing action are so highly interrelated, simply adjusting the neck is a preliminary step. I always do this in front of the client to show how much the guitar can be improved (and how well the mechanism works) and why the other issues of fret wear, bad neck angle, neck twist, popped fret(s), bridge height, etc., must also be addressed in a complete setup.

Since most classical guitar necks are not adjustable the corrections must be made in the wood by using heat and pressure or replaning and refretting and/or a combination of both approaches.  

If you can't make sense of this description then it is best to take the instrument directly to a qualified guitar repairer. Unless there is a structural problem (loose bridge, etc.) leave the guitar fully strung so it can be playable when examined. Without the normal pressure exerted by strings it is difficult for a repair technician to assess the guitar. Adjusting necks is not a big deal and I usually don't charge for it although it is polite for the customer to buy a set of strings for my time.

For any guitar, only an experienced repairer can methodically sort out the variables and determine the most appropriate resolution.  You should receive a clear explanation of your guitar neck's condition, including it's good points as well as the liabilities, followed by a brief step-by-step description of the procedure leading to the exact results.  If the repairer cannot efficiently deliver this information the person lacks experience or really doesn't know how the job will turn out.

Acquiring the observational skills oulined in this article will help you understand what must be done to restore your neck and fingerboard to optimal playing condition.  If you are in need of the services of a repairer such knowledge can only serve to assist both parties toward the goal of achieving mutual satisfaction.

Roger Thurman

Nominal String Height - General low settings for comfortable action

(At the 12th fret, measure with a fine ruler the gap from the top of the fret to underside edge of the string.)

Classical Guitar

First String (highest string): 1/8" or 3.2 mm.
Sixth String (lowest pitch): 5/32" or 4.0 mm.

Acoustic Steel String Guitar(six or 12-string)

First String: 1/16" or 1.6 mm.
Sixth String: 3/32" or 2.4 mm.

Electric Guitar

First String: 3/64" or 1.2 mm.
Sixth String: 1/16" or 1.6 mm

Flamenco Guitar

First String: 5/64" or 2.0 mm.
Sixth String: 3/32" or 2.4 mm

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Thurman Guitar & Violin Repair, Inc.
900 Franklin Av.
Kent OH 44240
Toll Free: 888-803-8693