"AF Envelope" is an envelope for protecting materials from light, dust, and acidic environments which can lead to deterioration of materials. Gusseted bottom and sides for storage of bulk and thick books. It also has a dust flap to prevent dust and other particles from entering the envelope. Great for sorting and organizing materials For a more secure way to store your documents when used with an acid free storage box. The original paper of the envelopes is made of a neutral protective paper Protect H (made by Tokai Paper Co., Ltd.) There are other types of AF envelopes. For general documents, used for old documents, photo use (with flap), A3