passe-partout

passe-partout (pas-pahr-TOO, pahs-par-TOO) - n., a light picture frame (mat) of cardboard, wood, etc. put between the picture and the glass; a method of framing in which the backing, a picture, a mat, and a glass are held together by strips of paper or cloth pasted over the edges.



Thanks, WikiMedia!

To give the meaning that's relevant to The Blue Castle -- originally, it meant a safe-conduct (in French literally "pass-everywhere") or by extension a master-key. Exactly how this metaphor got extended to framing mat, used both as added decoration and to separate the picture from the glass, is obscure to me.

---L.