passe-partout

noun, plural passe-par·touts [pas-pahr-tooz; French pahs-par–too] /ˌpæs pɑrˈtuz; French pɑs parˈtu/.

1. something that passes everywhere or provides a universal means of passage.
2. a master key; skeleton key.
3. an ornamental mat for a picture.
4. a method of framing in which a piece of glass is placed over a picture and is affixed to a backing by means of adhesive strips of paper or other material pasted over the edges.
5. paper prepared for this purpose.

noun

1. a mounting for a picture in which strips of strong gummed paper are used to bind together the glass, picture, and backing
2. the gummed paper used for this
3. a mat, often decorated, on which a picture is mounted
4. something that secures entry everywhere, esp a master key

n.“master-key,” 1670s, French, literally “pass everywhere,” from passer “to pass” (see pass (v.)) + partout “everywhere,” from par “through” (see per) + tout “all.”