Series Completion: AZ, GT, MN, ?, YB

Concept:

In an alphabet letter-pair series, each two-letter term is generated from the previous term by applying a FIXED number of forward and backward alphabet-position shifts: the first letter of a term steps forward by a constant count, and the second letter steps backward by the same constant count (or vice-versa), to produce the next term. Once that constant shift is found from any two consecutive given terms, every other term - including a missing one - can be generated or independently checked by applying it in sequence.

Application:

1. Assign each letter its alphabet position: A = 1, B = 2, ..., Z = 26.

2. First letters of the visible terms: A (1), G (7), M (13), and Y (25) in the last term.

3. Consecutive differences among first letters: G - A = 6, and M - G = 6, so the first letter steps forward by a constant 6 each time.

4. Second letters of the visible terms: Z (26), T (20), N (14), and B (2) in the last term.

5. Consecutive differences among second letters: T - Z = -6, and N - T = -6, so the second letter steps backward by a constant 6 each time.

6. Apply both constant shifts to the third term (M, N) to get the missing term: M (13) + 6 = 19 -> S; N (14) - 6 = 8 -> H. The missing term is SH.

Cross-check:

Apply the same two shifts once more, starting from the derived term (S, H), and confirm the result lands on the given final term: S (19) + 6 = 25 -> Y; H (8) - 6 = 2 -> B. This reproduces the given "YB" exactly, independently confirming that SH is the correct missing term.