Number 11 regular envelopes are slightly larger than the standard business #10 envelope. If you want to use the number 10 as return/reply you will want this envelope. A number 10 will fit inside a number 11. It has more room then the number 10 allowing for additional folded pages. It is small enough to qualify as a letter so it will not cost extra for postage based on size. Other common uses for the number 11 include using them for patient records and media kits.
