James Watson used cardboard cutouts representing the shapes of the DNA bases to figure out how bases pair. He realized that the adenine-thymine and cytosine-guanine pairings fit all the dimensions. (DNAi Location: Code > Finding the structure > Pieces of the puzzle > Watson's base pairing)

The Cavendish shop was to build us some tin models and that took too long, and, you know, finally in desperation, I made some out of cardboard. I began moving them around and I wanted an arrangement, you know, where I had a big and a small molecule. So, how did you do it? Somehow you had to form link bonds. So, here is A and here's T, and I wanted this hydrogen to point directly at this nitrogen so I had something like this. So then I went to the other pair. I wanted this nitrogen point to this one and it went like this. They looked the same. And you could put one right on top of the other. We knew if we just, even if we go up to the ceiling, we're building a tiny fraction of a molecule. Hundreds of millions of these base pairs in one molecule, all fitting into this wonderful symmetry, which we saw the morning of February 28, 1953.

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