We've needed something to take 8.5x11 sheets safely through the USPS. These envelopes do the job well. In a whole box of envelopes I believe we had only one or maybe two that failed to seal. That happened when we were trying to send a large number of folded sheets together in one envelope. The packet was almost too large to fit in the envelope. We resolved the issue by using some clear plastic sticky tape that was the 2" wide sort over the back of the envelope lengthwise. The lawyers had no problem opening the packet.