Envelopes come in a variety of sizes. Security envelopes have extra thickness so that documents cannot be read through the envelope by holding it up to a light source. Windowed envelopes have a clear plastic window that allows an address to show through. Typical envelope sizes include:
No 6 —3 5/8 inches x 6 1/2 inches
No 9—3 7/8 inches x 8 7/8 inches
No 10—4 1/8 inches x 9 1/2 inches