#9 Blank Envelopes are one of the most popular commercial-sized envelopes available, Commonly used as a reply envelope that can be sent along inside a #10 regular envelope for your clients to use to mail back to the sender. This 24lb. Bright White is a quality and economical choice. It's available both plain or custom printed for branding, address printing services are also available. These #9 size envelopes are also machine-insert able for higher-volume direct mail. Sealed by a moist enable glue on the back flap.