A picture is a connected set of axis parallel unit lines from the Cartesian plane considered as a square grid. A word over the alphabet {l,r,u,d} is a picture description in the sense that it represents a traversal of a picture where the interpretation of the symbols l,r,u,d is: l — go (and draw) one unit line to the left of the current point; r,u, and d are interpreted analogously with “left” replaced by “right”, “up”, and “down”, respectively. This is one of the chain codemodels.