|00:51In about 1930, shortly after steel strings had become standard, Martin designed a new bridge to handle the higher tension of the steel strings. The belly bridge is about 1-1/2 inches wide at the center, while the older rectangular "pyramid" bridges were only one inch wide. This modern bridge is called the "belly bridge" because of the bulging area behind the bridge pins.
Not to be left out, Gibson redesigned their bridges on several models, and came up with a "reverse belly" bridge variant which has the entire row of bridge pins right at the back edge, allowing almost no gluing surface where it is needed most. So they use a couple of small bolts, hidden under the pearl inlay dots directly in line with the bridge pins. Even with the bolts, these bridges are not nearly as stable as the Martin design.