Sketchpad

Sketchpad was a revolutionary computer program written by Ivan Sutherland in the course of his PhD thesis, for which he received the Turing Award. It helped change the way people interact with computers. Sketchpad is considered to be the ancestor of modern computer-aided drafting programs as well as a major breakthrough in the development of computer graphics in general. 

Sketchpad was the first program ever to utilize a complete graphical user interface, using an x-y point plotter display and the recently invented light pen. The clever way the program organized its geometric data pioneered the use of in computing and pointed forward to object oriented programming.

The main idea was to have master drawings which one could instantiate into many duplicates. If the user changed the master drawing would change as well. Another major invention in Sketchpad was that it constrain geometric properties in the drawing the length of a line or the angle between two lines could be fixed. 

Sketchpad is considered to be the ancestor of modern computer-aided drafting programs as well as a major breakthrough in the development of computer graphics in general. For example, the Graphic User Interface was derived from the Sketchpad as well as modern object oriented programming.