Like the story line in a play or the principal phrase in a musical composition. Effective design is directed from a single narrative.
I try to have that 'one thing " that defines the design - the rest of the elements support it in sympathy. It's the hinge pin that holds the image together.
This small piece on top of a city farm sign "made" the design. without it the design becomes pedestrian. A small piece - without it the structure lacks anchorage.