Robert Riddell『The Carpenter and Joiner, Stair Builder, and
Robert Riddell『The Carpenter and Joiner, Stair Builder, and,Elevator Enclosures and Cars,Impossible Constructions – M.C. Escher – The Official Website,Circular Work in Carpentry and Joinery: Collings, George,Paul Rudolph at the Metropolitan Museum of Art, in New York