 The difference is important for the long term protection of the piece because acidic mats can cause what is called "mat burn", brown marks that creep in from the outside onto the displayed piece itself.While mat burn is sometimes reversible through cleaning the piece, cleaning may not be feasible if the piece was executed in water-soluble inks or paints, such as watercolor.Thus, it is important to know if the mats used are acid-free if the piece is to be preserved for a long time.