 Typically, such items are instead held in place against the backing with mylar "photo corners" (tiny triangular pockets into which the corners of the paper are put).The added (though slight) weight of mats can help hold a piece in place while also helping to hide the backing and photo corners.In archival framing, the mats are not glued to the piece or backing, but are "hinged" to the backing with tape, though if more than one mat is used, the mats are typically glued to each other.