If a straight line falling across two straight lines makes the alternate angles equal to one another then the (two) straight lines will be parallel to one another. * For let the straight line $EF$, falling across the two straight lines $AB$ and $CD$, make the alternate angles $AEF$ and $EFD$ equal to one another. * I say that $AB$ and $CD$ are parallel.
If a straight line \((EF)\) intersects two straight lines \((AB)\), \((CD)\) such that the alternate angles are equal \((\angle{AEF}=\angle{DFE})\), then these lines are parallel \((AB\parallel CD)\).
Proofs: 1
Proofs: 1 2 3 4 5
Propositions: 6