A program that allowed users to draw on a screen with a {light pen}.It supported constraints (e.g. drawing a constrained ellipse produced a circle).It also had some {computer aided design} features (e.g. computing loads on beams).Sketchpad was the subject of Ivan E. Sutherland' s 1963 MIT PhD thesis, which opened the field of computer graphics.It was the progenitor of computer drawing packages like MacDraw or Adobe Illustrator.There is a film of Sketchpad in action.It solved constraints using value inference and introduced the "ring" list structure.["Sketchpad: A Man-Machine Graphical Communication System", I.e. Sutherland, MIT Lincoln Lab, TR 296 (Jan 1963)].[Sammet 1969, p. 678].(1995-02-14)