The pairs are: O-R (2 letters in word, 2 in alphabet), P-R (1 letter in word, 1 in alphabet), and A-E (2 letters in word, 3 in alphabet)... Checking all pairs: O-R (COR), P-R (POR), R-T (RAT), A-E (ATE). There are three such pairs: P-R, O-R, and R-T.