The connecting procedure for Y and X

Any arrow a ← b in the quiver of A yields corresponding maps
      P(a) → P(b) (between the indecomposable projective modules) and
      Q(a) → Q(b) (between the indecomposable injective modules).

Let y be a vertex of the quiver of A, such that P(y) is not a direct summand of T.
Then we get an Auslander-Reiten sequence:
 
Here, F = HomA(T, - ), F' = Ext1(T, - ). The direct successors of y are labelled x, the direct predecessors are labelled z.

On the left, the indecomposable projective modules are encircled,
thus AA,
on the right the indecomposable injective modules are encircled,
thus D(AA).