Yea, like the answer above, the sequence is going from first letter to last. For example, A is the first letter of the alphabet, and Z is the first-last letter of the alphabet. Therefore we have AZ. B is the second letter of the alphabet, and Y is the second-last letter of the alphabet. Therefore we have BY. C is the third letter of the alphabet, and X is the third-last letter of the alphabet. Therefore we have CX. Now D is the fourth letter of the alphabet, and W is the fourth-last letter of the alphabet. Therefore we finally have DW.