passe-partout

That by which one can pass anywhere; a safe-conduct.

A master key.

The traditional (or modern) way of confronting complexity was to find a secure point of reference that could serve as foundation, a passe-partout, a master key from which everything else could be derived.

A light picture frame or mat of cardboard, wood, etc., usually put between the picture and the glass, and sometimes serving for several pictures.

A new introduction for use in conjunction with passe-partout framing is a series of corner pieces which are readily folded round the corners of the finished passe-partout, giving it a certain added effect and slight embellishment.