From the September/October 2019 issue of Acoustic Guitar | BY MARTIN KEITH

Q: I would like an opinion on strings for my 1936 Martin D-18. Medium-gauge strings sound best, but sometimes I worry about the neck! Will mediums put too much pressure on it over time? If I do use mediums, would it be good to loosen them some when not playing the instrument? Dave, via e-mail

A: Congratulations—a 1936 D-18 is quite an instrument! Martin records indicate that only 258 of these guitars were built that year, and many haven’t survived. Your guitar features the forward-shifted X pattern that has become legendary. For all these reasons, it’s gratifying to hear that you take its care seriously, as too many of these historic guitars have gone to early graves.

Before we get to the string gauge question, let’s take a quick look at how string tension usually affects a flattop guitar—bending the neck and distorting or “bellying” the top. Both can result in high action, so it’s important to know which is which when evaluating a vintage instrument. They require different techniques to correct, and can have different implications for the long-term health of the instrument.

A bowed neck is easy to diagnose. Stop a string at the first fret with your fretting hand, and use your other hand to stop the same string at the 14th fret. This positions the tensioned string as a straight-line reference, which can be used to determine neck curvature. Look at the seventh- and eighth-fret positions to see how much space there is between the fret tops and the underside of the string. For most playing styles, a gap somewhere around the thickness of a business card is ideal. If the neck has a significant forward bend, a competent luthier can use a variety of techniques to straighten it out, including heating or refretting with larger-tang fretwire. 

With a modern guitar, dialing in the proper neck relief requires no more than a simple truss-rod adjustment. Your D-18 has a nonadjustable, T-shaped steel stiffening bar in the neck, which most vintage Martin experts agree is strong enough to support string tension over the long term. If there were a particular weakness in your guitar’s neck, it would most likely already have bowed by now.

Bellied tops are common in old flattops, and are not necessarily a sign of weakness—most guitars that are built lightly enough to be powerful and responsive will exhibit some degree of bellying, especially after 80 years! However, a bellied top can sometimes indicate larger issues, such as loose braces, which can lead to cracks, wrinkles in the top, and other existential threats, so proper diagnosis is important.


On a new instrument, it’s typical for the top of the bridge (not the saddle, but the bridge itself) to be in plane with the fret tops. So, if you lay a ruler on the frets, it should land right on the surface of the bridge. Though this varies with specific makers and models, average height of the bridge is about 3/8 inches from the surface of the top, with the saddle approximately 1/8 inch above that.

As the top distorts over time, the bridge rises, and the action goes up with it. The common remedy is to lower the height of the bridge saddle. This does get the action back down again, but it also affects an important variable in the guitar’s response—the amount of torque (rotational force) the strings exert on the top. A high saddle puts greater torque on the top, and each guitar has an ideal amount that will maximize volume and responsiveness. Lowering a saddle too far reduces this torque and can sometimes result in a loss of power and sound quality.

On older guitars with substantially bellied tops, it’s common to see bridges that have been cut lower to steal a bit of extra clearance and further lower the saddle. This is a poor approach for many reasons. It further worsens the torque problem and also weakens the bridge, which is an important structural support component of the top. In fact, shaving down a bridge—and thereby making it more flexible—can increase the likelihood of top distortion in the future.

Restoring a bellied guitar to proper playability is a job for a qualified luthier, and usually involves a neck reset—removal of the neck using steam, and re-cutting the joint to increase the neck angle, which repositions the fret surface into proper alignment. A good reset will be nearly invisible and will usually preserve or even increase resale value of a vintage instrument, rather than detracting from it. 

Bellied tops can also sometimes be flattened using heat and moisture, and this approach has been proven successful as a less-invasive alternative to a full reset. Restoration expert TJ Thompson’s effective tools and techniques for fixing bellied tops with heat and moisture are gaining popularity in repair shops.

Returning to the string gauge question: for most manufacturers, medium refers to the set gauge of .013–.056. The overall tension on an average medium set is around 185 lbs., compared to 160 for a light set (typically .012–.054)—a difference of about 14 percent. By comparison, tuning your 13s down to D (one whole step) lowers the tension to 145 lbs., or about 20 percent. (Thanks to D’Addario’s for supplying these numbers.) So, if you tune down between playing sessions, mediums in D will put less stress on the guitar than light-gauge strings at standard pitch. (That D-18 probably sounds great tuned down a step, as well!) Another good option would be a hybrid set of mixed light/medium gauges, which could keep the tension lower while still giving you the sound you’re after.

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To further reassure you: If the saddle has been lowered on your guitar in the last 80 years, and it likely has, then the torque on the top will have been reduced, too, which lowers the risk. This may be why you prefer the sound of the medium-gauge strings—they may put a load on the top that is comparable to what light-gauge strings would produce with a taller saddle.

To answer your other question, in general, I don’t think it’s necessary to tune down for storage, provided the guitar is structurally sound. Frequent rapid changes of any kind (humidity, temperature, or tension) are best avoided with vintage instruments. But if you do choose to tune down, a whole-step drop should be more than enough.

Each year we lose a few more vintage guitars to damage, neglect, and bad repair work. We are stewards of these instruments, and with proper care, they can and should outlive us. It sounds like your D-18 is in good hands.

Martin Keith is a luthier, repair and restoration expert, and working musician based in Woodstock, New York. 

This article originally appeared in the September/October 2019 issue of Acoustic Guitar magazine.