These #9 double-window envelopes accommodate most 8.5-by-11-inch computer-generated statements and invoices. Since business correspondence often includes personal account or client information, Quality Forms invoice envelopes offers the ultimate in protection, privacy and confidentiality when mailing invoice documents. The envelopes features an inner security tint that prevents documents from being read through the envelope when held up to the light. The woven paper can stand up to significant wear and tear, so there's no need to worry about the envelope ripping open. The #9 envelopes are standardized to suit common business invoice layouts.