GT : M :: CN : ? Which letter completes the letter analogy?

The smaller position value is taken from the larger, and the difference names the answering letter. G is 7th and T is 20th, and 20 − 7 = 13, which is M. C is 3rd and N is 14th, and 14 − 3 = 11, which is K. 'Q' comes of adding the two positions, 3 + 14 = 17, but adding on the first pair gives 27, past the last letter altogether, so that rule cannot even be applied there. J is 10th and L is 12th, the positions flanking the eleventh.