In the original story, NASA finds a glowing diamond-like structure on the Moon. For various reasons, Kubrick decided to go with something else. They edited the storyboards and put a black rectangle over the diamond. The rectangle was a symbolic TBA. One day they were looking at the boards and realized that the monolith would actually look very cool.
Years later, after thousands of speculations, a fan approaches Arthur C. Clarke and tells Clarke he’s unravelled the mystery. The ratio of the rectangle is 1 : 4 : 9; those are the squares of the first three numbers. Clarke liked it so much he used it himself.