Button-and-String Letter Envelopes, Navy, Set of 10These are #10 envelopes the most common business size, and the correct choice for an 8. 5" x 11" letter folded in thirds. So you can put anything in these stylish envelopes that you'd normally mail in a standard envelope: letters, brochures (up to about 15 sheets), etc. Each envelope is fitted with a button and string closure that can be used many times. Inside, the flap is also gummed in case you want to use it for mailing. Ten envelopes per pack.