In the Martin naming system you always have first the size and then the style. So a D would be a Dreadnought size guitar in the 18 style, while the OM would be an orchestra model guitar in the 28 style. While the letter in the name refers to a body size and shape, the number always refers to a style type. The style type tells you about the specific tonewoods and appointments that have been put into a particular style of guitar.
Download includes PDF continuous document allowing you to see or print the entire catalog. Also includes 2-page section on Martin Instrument Specifications. PDF is 2 pages allowing you to see or print this historical promo pamphlet clearly. Great reference material.