The 6 1/2 Booklet envelope has a width of 9 inches and a height of 6 inches, giving it a total surface area of 54 square inches. Its maximum enclosure size is 5.75 x 8.75 inches. This booklet envelope is sometimes referred to as the 6-1/2 catalog envelope, due to its sizing.
Booklet envelopes like the 6 1/2 get their dimensions from the height and width in inches rather than a numbering system. The 6 1/2 booklet is designed with an open side seam to allow easy insertion of documents without folding. The added width compared to the 6 booklet provides extra room for contents like small catalogs, pamphlets, brochures and documents requiring flat, protected mailing. The open side reduces wear from folding. With dimensions between the smaller 6 and larger 9 booklet envelopes, the 6 1/2 offers an incremental size increase for booklet mailings.