The us-envelope Series

Further Information

There are dozens of sizes of envelopes available. The designations such as 'A2' do not correspond to ISO paper sizes. (Often, North American paper jobbers and printers will insert a hyphen to distinguish from ISO sizes, thus: A-2.). Envelopes accepted by the U.S. Postal Service to be eligible for mailing at the price for letters must be rectangular and at least  3 1/2 inches high × 5 inches long × 0.007 inch thick.

Source: Wikipedia