Question

What is the next letter a z b y c x d

Answer 1

The next logical letter would be w.
This pattern starts at the first letter, then the last letter, then the second letter, then the second to last, the third, third to last, and so on.

Answer 2

The next letter is w.

a, __, b, __, c, __ d, __
_, z  , _,  y , _,  x ,_, W

get the picture?