Number 10 window envelopes are the most popular size used in business. They are commonly used for invoices, statements and letters or anything that tri-folds down from an 8.5x11. These are also a standard envelope for payroll checks. The window in this #10 allows the address of the recipient to show through. These envelopes can be completely customized with your logo and return address. Order these # 10 windows for your custom envelope business office mailing needs.