It will be convenient first to fix a form of notation for the corners of the cube. With the origin within the cube and rectangular co-ordinates parallel to three edges meeting in a corner, the eight corners lie each in one of the eight octants, and may conveniently and symmetrically be represented thus:—

Or, calling any corner A, the corners distant √2 from A (the length of the edge being 1) and taken positively, i.e., contrary to the watch-hand way, as seen from A, are B, C, D (see figures).
So that, passing from corner to corner along an edge, we change both letter and sign, along a face diagonal we change letter but not sign, along a body diagonal we change sign but not letter.