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)
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) ↩