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Douglas Harper's Etymology Dictionary
passe-partout

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

Wiktionary
passe-partout

n. 1 (context obsolete English) That by which one can pass anywhere; a safe-conduct. 2 (context obsolete English) A master key; a latchkey. 3 (context obsolete English) A light picture frame or mat of cardboard, wood, etc., usually put between the picture and the glass, and sometimes serving for several pictures.

WordNet
passe-partout
  1. n. key that secures entrance everywhere [syn: passkey, master key, master]

  2. a mounting for a picture using gummed tape

Wikipedia
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.

Usage examples of "passe-partout".

In fondo a un atrio tutto marmi e indorature sedeva un portiere in unifor­me, che salutò i detective e fornì loro una serie di passe-partout.