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What is the difference between a letter and a flat?

To qualify as a letter, a mailpiece must be a minimum of 3 1/2" H x 5" L and .007 inch thick, but not larger than 6 1/8" H x 11 1/2" L and 1/4" thick. A flat is a mailpiece larger than 6 1/8" H x 11 1/2" L and 1/4" thick but smaller than12" H x 15" L and 3/4" thick.