Hitchcock: McGuffin

The Mcguffin is an irrelevant object (nothing at all), around which the action is structured

Certain function of certain object

McGuffin signifies only that it signifies

They signify the signification as such, but the actual content is insignificant.

39 Steps: plans for airplane machine (we never see the airoplane)

Foreign Correspondent:

secret clause in a defense treaty, so secret that it had  to be memorized by those present. It could not even be put on paper; an ideal signifier beyond writing.

Lady Vanishes:

The tune

NBNW: the microfilm