If quadrilateral abcd is congruent to quadrilateral efgh, then what corresponding side is congruent to side ef?

When writing congruent quadrilaterals, the order of writing the letters is very important. This is because, each side/angle in the first quadrilateral is congruent to its corresponding side/angle in the second.

Now, we are given that:
ABCD is congruent to EFGH, the letters corresponsponding to EF in the other quadrilateral is AB.
Therefore,
EF is congruent to AB