These Polypropylene Photo Sleeves are sealed on 2 parallel (longer) sides and open on 2 (shorter) sides. They are archival, made of cast polypropylene. They are clear and easy to read through. They are made of a heavier polypropylene than our envelopes, and feature a fold-locking closure. The fold lock is nice for two reasons. First, if the photo or paper item is fragile, you can unlock the protector and gently remove the item. In any other protector, you need to slide the item in and out - thus putting stress on the item unnecessarily. Second, the fold lock provides extra durability and sturdiness to the enclosure. We have found these sleeves to be the best sleeve to use for fragile items because you do not need to slide the item into the enclosure.