How did Watson and Crick figure out the nitrogenous bases pairing

- At first, Watson and Crick thought the bases paired like with like (A with A, and so on), but such pairings did not result in a uniform width ​ - Instead, pairing a purine (A or G) with a pyrimidine (C or T) resulted in a uniform width consistent with the X-ray data​ - They determined that adenine (A) paired only with thymine (T), and guanine (G) paired only with cytosine (C)​