Another innovative feature for that time was the user interface combining a keyboard with a monitor and light pen used for correcting texts and drawing on screen.
As the tip of the light pen makes contact with the screen, it sends a signal back to the computer containing the coordinates of the pixels at that point.
Sketchpad's light pen drew directly on a monitor, and the program locked in right angles, arcs and other shapes to help create detailed, accurate and replicable illustrations.