والأين: هي نسبة المتمكن إلى المكان.
Place is the relation of a located thing to its location.
Zayd is in the house — this identifies his relation to a place.
The bird is on a tree — this gives its location relative to the tree.
Ayn answers “Where?”. Saying where Zayd is does not change his essence, length or color; it describes his spatial relation.