Consider parallel lines AB, CD and EF as shown in the figure.
XY and MN are known as transversals
$$\angle $$ XPQ = $$\angle $$ PRS = $$\angle $$ RTU as corresponding angles are equal
Interior angles on the side of the transversal are supplementary. i.e. $$\angle $$ PQS + $$\angle $$ QSR = 180 degrees
Exterior angles on the same side of the transversal are supplementary. i.e. $$\angle $$ MQB + $$\angle $$ DSU = 180 degrees.
Two transversals are cut by three parallel lines in the same ratio i.e. PR / RT = QS / SU
