A good short explanation.............google is the best.
Pre-war era Martins have a bracing pattern that many enthusiasts believe to be the best. In truth, Martin did two things differently. First, braces were scalloped in that era. That is, wood was selectively removed from certain areas of the braces to weaken the top enough to allow it to vibrate freely without weakening it so much as to make it structurally unsound. Scalloping opens up the mids and increases volume. Second, Martin used a forward shift of their X bracing. On most X-braced steel string guitars, the "X" crosses approximately 2" below the soundhole. On pre-war Martins, the "X" crosses about 1" below the soundhole. The result is that the bridge rests less directly atop the main X-braces and transfers more of its vibration to the top.
In the 1940s, Martin moved the X bracing away from the soundhole to its current position and quit scalloping braces. According to Martin, they did this due to the preponderance of players using heavy -gauge strings to boost their volume. The light bracing pattern coupled with heavy strings resulted in a high damage rate. Because pre-war Martins are becoming so rare and expensive, Martins along with many other builders are now producing guitars with scalloped and forward-shifted bracing.
Pre war bracing is also what Martin uses on its Marquis, Golden Era and Authentic models as well as select signature models and limited editions, i.e. it is very popular... my 3 go to Martins all have pre war bracing, so it appeals to my old ears.
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