Staples Fold-and-Stick #10 Double Window Security Envelopes - 4-1/8" x 9-1/2" - White - 250 PackSend important mail with confidence with Staples Fold and Stick #10 Double Window Security Envelopes. Ideal for business correspondence, #10 is a commonly used envelope size for mailing letters, cheques, billing statements, invoices and more. Quick and easy to use, they feature a fold and stick closure that doesn't require moisture. Simply raise the lower flap and press it to the upper flap to seal. The white envelopes are designed with a square flap