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W, X, Y and Z are the vertices of the parallelogram in order,
so W and Y are opposite each other,
which means that WY is a diagonal of the parallelogram.
Also XW and YZ are parallel sides of the parallelogram.
So diagonal line WY intersecting the two parallel lines XW and YZ,
and we call line WY a transversal line intersecting two parallel lines.
Angles a transversal makes with the two parallel lines that are
1) the two parallel lines, and
2) on sides of the transversal
are called angles,
and are congruent to each other.
Angles and are congruent because they are alternate interior angles.