Sketchpad is a man-machine graphical communication system developed by Ivan Edward Sutherland. It uses drawing as a novel communication medium between a computer and a human. The system includes input, output, and computation programs that enable it to interpret information drawn directly on a computer display. It has been used to draw various types of drawings, including electrical, mechanical, scientific, mathematical, and animated ones. Sketchpad is a general-purpose system that is particularly useful for understanding processes such as the motion of linkages, which can be described with pictures. It also makes it easy to draw highly repetitive or highly accurate drawings and to change previously drawn drawings. Sketchpad allows users to sketch directly on a computer display using a "light pen." The light pen is used both to position parts of the drawing on the display and to point to them to change them. A set of push buttons controls the changes to be made, such as "erase" or "move." Except for legends, no written language is used. Information sketched can include straight line segments and circle arcs. Arbitrary symbols may be defined from any collection of line segments, circle arcs, and previously defined symbols. A user may define and use as many symbols as he wishes. Any change in the definition of a symbol is at once seen wherever that symbol appears. Sketchpad stores explicit information about the topology of a drawing. If the user moves one vertex of a polygon, both adjacent sides will be moved. If the user moves a symbol, all lines attached to that symbol will automatically move to stay attached to it. The topological connections of the drawing are automatically indicated by the user as he sketches. Since Sketchpad is able to accept topological information from a human being in a picture language perfectly natural to the human, it can be used as an input program for computation programs which require topological data, e.g., circuit simulators. Sketchpad itself is able to move parts of the drawing around to meet new conditions which the user may apply to them. The user indicates conditions with the light pen and push buttons. For example, to make two lines parallel, he successively points to the lines with the light pen and presses a button. The conditions themselves are displayed on the drawing so that they may be erased or changed with the light pen language. Any combination of conditions can be defined as a composite condition and applied in one step. It is easy to add entirely new types of conditions to Sketchpad's vocabulary. Since the conditions can involve anything computable, Sketchpad can be used for a very wide range of problems. For example, Sketchpad has been used to find the distribution of forces in the members of truss bridges drawn with it. Sketchpad drawings are stored in the computer in a specially designed "ring" structure. The ring structure features rapid processing of topological information with no searching at all. The basic operations used in Sketchpad for manipulating the ring structure are described. Thesis Supervisor: Claude E. ShannonSketchpad is a man-machine graphical communication system developed by Ivan Edward Sutherland. It uses drawing as a novel communication medium between a computer and a human. The system includes input, output, and computation programs that enable it to interpret information drawn directly on a computer display. It has been used to draw various types of drawings, including electrical, mechanical, scientific, mathematical, and animated ones. Sketchpad is a general-purpose system that is particularly useful for understanding processes such as the motion of linkages, which can be described with pictures. It also makes it easy to draw highly repetitive or highly accurate drawings and to change previously drawn drawings. Sketchpad allows users to sketch directly on a computer display using a "light pen." The light pen is used both to position parts of the drawing on the display and to point to them to change them. A set of push buttons controls the changes to be made, such as "erase" or "move." Except for legends, no written language is used. Information sketched can include straight line segments and circle arcs. Arbitrary symbols may be defined from any collection of line segments, circle arcs, and previously defined symbols. A user may define and use as many symbols as he wishes. Any change in the definition of a symbol is at once seen wherever that symbol appears. Sketchpad stores explicit information about the topology of a drawing. If the user moves one vertex of a polygon, both adjacent sides will be moved. If the user moves a symbol, all lines attached to that symbol will automatically move to stay attached to it. The topological connections of the drawing are automatically indicated by the user as he sketches. Since Sketchpad is able to accept topological information from a human being in a picture language perfectly natural to the human, it can be used as an input program for computation programs which require topological data, e.g., circuit simulators. Sketchpad itself is able to move parts of the drawing around to meet new conditions which the user may apply to them. The user indicates conditions with the light pen and push buttons. For example, to make two lines parallel, he successively points to the lines with the light pen and presses a button. The conditions themselves are displayed on the drawing so that they may be erased or changed with the light pen language. Any combination of conditions can be defined as a composite condition and applied in one step. It is easy to add entirely new types of conditions to Sketchpad's vocabulary. Since the conditions can involve anything computable, Sketchpad can be used for a very wide range of problems. For example, Sketchpad has been used to find the distribution of forces in the members of truss bridges drawn with it. Sketchpad drawings are stored in the computer in a specially designed "ring" structure. The ring structure features rapid processing of topological information with no searching at all. The basic operations used in Sketchpad for manipulating the ring structure are described. Thesis Supervisor: Claude E. Shannon