The issue of using X-bracing on a carved-soundboard/backboard mandolin comes up often and is a topic worthy of a lot of discussion.

X-bracing was originally designed for flattop steel string guitars as a means of counteracting the +/- 160 pound torque-load of the strings at the bridge, and keeping the soundboard from distorting under the load. The two lower arms of the "X" straddle the bridge plate (the cross-grain support piece inside a guitar that is directly under the bridge) and prevent that area of the soundboard from twisting out of shape, and the two upper arms extend around and beyond the soundhole. Ideally, the soundboard should have distortion around the bridge; it is a sign that the soundboard is properly loaded by the strings' tension and ready to be responsive to the longitudinal energy driven by the strings towards the bridge to make it rock back and forth. The idea of X-bracing was initiated by C.F Martin I and was embellished upon by many manufacturers and private builders since.

While the X-brace system works well for a flattop acoustic guitar from a structural standpoint, there are a lot of things about the X-brace method that is not ideal from an acoustic standpoint. (The ideal brace pattern for acoustic guitars, in my opinion, is the fan-brace pattern used in classical and Flamenco guitars.) If that comment raises some eyebrows or conjures up the question, "What's wrong with an old herringbone D28?" I would answer, "Absolutely nothing - after all, it is the standard by which other guitars are measured." But the fact that an old D28 sounds great also means that it is a sound we've become accustomed to and one we identify with bluegrass and with specific artists. It doesn't mean that it is the greatest sound that can be had from that body shape and style.

Unlike the flat soundboard of an acoustic guitar, the carved and arched soundboard of a mandolin is very strong because of its curved shape. In fact, an arched soundboard doesn't need braces; you can build an F5-style mandolin without any braces or tone bars and it will hold up just fine. Of course, since mandolins feature a tailpiece and a movable bridge, there is no torque or twisting load imposed on the center of the soundboard; the mandolin's soundboard is subjected to a downward load and doesn't need braces to keep it from twisting as a guitar's soundboard does.

Another important feature of a graduated soundboard is that it is stiffer in the center and becomes increasingly more supple near the edge of the soundboard.

Tone bars and braces are two very different things: braces are for structure; tone bars are to adjust the stiffness (tuning) of the soundboard. Therefore, the tone bars used on a mandolin soundboard are there to adjust the tuning of the soundboard, not to support or strengthen it. (F4 mandolins and H4 mandolas feature a cross brace below the soundhole, and it is there to strengthen the soundboard and prevent it from cracking where the oval soundhole is cut across the soundboard's grain.)                       

One structural feature of an X-brace system is that the center of the "X" is typically higher than the extensions or ends of the braces (Fig. 1). In this regard, it is a structure that is stiffer in the center and weaker at its extremities. These same attributes can be given to the arched and graduated soundboard of the mandolin (Fig. 2). However, to put X-bracing on a graduated soundboard doubly stiffens the soundboard at the center and makes it excessively stiff and heavy (Fig. 3).

If a builder was using a laminated soundboard on a mandolin, or a soundboard of even thickness across its entire width and length (Fig. 4), then an X-bracing system could be made to work.

Another concern I have for X-bracing on a mandolin soundboard has to do with the location of the bridge feet. Ideally, each of the two feet of the bridge should sit over the tone bars (as it does on an F5 mandolin - Fig. 5). Unfortunately, unless an X brace pattern is laid out in a very unusual way, but still intersects in or near the center of the soundboard, it is difficult to get the mandolin's bridge feet to sit over the legs of an X-brace system.