A passe-partout, put between the picture and frame, protects the picture and changes its visual appearance.
A framed daguerrotype surrounded by a passe-partout
Passe-partout (or passepartout) is the French term for a mat, a paper or, more usually, cardboard sheet with a cutout, which is placed under the glass in a picture frame. A picture (a photo or print, drawing, etc.) is placed beneath it, with the cutout framing it. The passe-partout serves two purposes: first, to prevent the image from touching the glass, and second, to frame the image and enhance its visual appeal. The cutout in the passe-partout is usually beveled to avoid casting shadows on the picture. The word may also be used for the tape used to stick the back of the picture to its frame. The term "mount" is used in British English.