If two planes cut one another then their common section is a straight line. * For let the two planes $AB$ and $BC$ cut one another, and let their common section be the line $DB$. * I say that the line $DB$ is straight.
(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) ↩