if the master drawing was changed then the changes would automatically be propogated through the instances in any duplicates. the master-instance concept allowed the creation of a master drawing and then duplicates to be created which would inherit properties of the objects in the master drawing unless they were locally changed,. an advanced memory architecture was developed that allowed the creation of master objects and "instances" which were very memory efficient copies of the masters,. 
rubber-banded lines could be constrained to always intersect at a precise angle,. the light pen was used to draw directly on the computer's monitor and incorporated graphical user interface techniques such as rubber-banding of lines and zooming,. Sketchpad's most incredible breakthroughs were in the way that it allowed the user to interact with the computer: The computer occupied about 1,000 square feet (~93 square meters) and the 320kb memory core alone was ~1 cubic yard (~0.76 cubic meters). 
As with most computers of that era, programs were written in macro-assembler, punched onto paper tape and fed into the computer's paper tape reader. The computer was very advanced for its time and had 320kb main memory, an 8Mb magnetic tape storage device, a 7 inch 1024x1024 monitor, a light pen and a button box. Sketchpad was developed at MIT's Lincoln Laboratory on a TX-2 computer. It is a tribute to Sketchpad's uniqueness that it defined a GUI (Graphical User Interface) more than 20 years before the term was first used. Sketchpad was an innovative system developed in 1963 by Ivan
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