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)
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) ↩