If a straight-line stood on a(nother)  straight-line makes angles, it will certainly either make two right-angles, or (angles whose sum is) equal to two right-angles. For  let some straight-line $AB$ stood on the straight-line $CD$ make the angles $CBA$ and $ABD$. I say that the angles $CBA$ and $ABD$ are certainly either two right-angles, or (have a sum) equal to two right-angles. <prf> Thus, (the sum of) $ABD$ and $ABC$ is also equal to two right-angles.Thus, if a straight-line stood on a(nother)  straight-line makes angles, it will certainly either make two right-angles, or (angles whose sum is) equal to two right-angles.