If two straight lines cut one another then they are in one plane, and every triangle (formed using segments of both lines) is in one plane. * For let the two straight lines $AB$ and $CD$ have cut one another at point $E$. * I say that $AB$ and $CD$ are in one plane, and that every triangle (formed using segments of both lines) is in one plane.
(not yet contributed)
Proofs: 1
The proofs of the first three propositions in this book are not at all rigorous. Hence, these three propositions should properly be regarded as additional axioms (translator's note) ↩