I have been using the Staples brand of security envelopes for 6 years now. I have to mail out hundreds of envelopes between Dec and the end of August for a summer camp. Sometimes only one or two sheets go into the envelopes but closer to the start of camp I am mailing out as many as 8 to 10 sheets of paper folded in thirds and they fit just fine in these #9 envelopes. I do use a glue stick to ensure they will stay closed but have never had a complaint about them arriving open. The size of the window is perfect for our pre printed invoices that show through the return as well as the going to addresses.