These .004 envelopes are sealed on three sides, and open on one long side to ease insertion. the .004 polyester offers extra rigidity and support for heavy or fragile documents. They are made of inert, acid-free polyester and are a popular choice for preservation of photos, negatives, letters, documents, drawings, and many other flat items. The size listed is the size which will fit inside the envelopes; they are slightly larger externally. Packages of 10.