I stuck two thin hardback books in one of these and took to the post office for mailing. I was randomly selected to have my package opened and checked for contents then resealed if it did indeed contain what I said it did —- books. Staff behind the counter had a helluva time trying to open it “nicely” without damage to that part of the envelope. And that proved impossible due to the strength of the seal. They did get it open but had to use a lot of tape to fix what their necessary force had done. So: very strong seal.