Equal triangles which are on the same base and on the same side are also in the same parallels.
Let ABC, DBC be equal triangles which are on the same base BC and on the same side of it;
[I say that they are also in the same parallels.] And [For] let AD be joined; I say that AD is parallel to BC. For, if not, let AE be drawn through the point A parallel to the straight line
BC, [I. 31] and let EC be joined. Therefore the triangle ABC is equal to the triangle EBC; for it is on the same base BC with it and in the same
parallels. [I. 37] But ABC is equal to DBC;
Similarly we can prove that neither is any other straight line except AD;
Q. E. D. 1