Martin D12-35's and D12-18/28's are braced heavier than the standard 6-string Martin D's. The Gibson D's have 'double-X' bracing and it's slightly heavier than the 6's of the same vintage. The B12-25's, while not 'double-Xed' have heavier bracing than the standard B25 6-string. Speaking of Bob Gibson there's a story of him tuning his guitar for an entire set without playing a song. Legend has it after hilarious observations, caustic remarks and general social commentary, at the end of the set he finally got his guitar in tune, strummed a C chord and left the stage to a standing ovation. Is this story true? I heard it years ago and always wondered.