The Diagram
One sign of a good design is if it's basic 'parti' can be explained in a simple diagram.
These thumbnails are often carried out freehand in ad-hoc circumstances, with an immediacy which is borne of limited time and materials. Their directness distills the complex thought processes behind them, clarifying the essential idea behind a project. They capture the creative 'moment' of design, in this case the moment a sample project for a £500m bid was cracked.