Proposition: Prop. 11.03: Common Section of Two Planes is Straight Line

Euclid's Formulation

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.


Modern Formulation

(not yet contributed)

Proofs: 1

Proofs: 1 2 3

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Adapted from (subject to copyright, with kind permission)

  1. Fitzpatrick, Richard: Euclid's "Elements of Geometry"

Adapted from CC BY-SA 3.0 Sources:

  1. Prime.mover and others: "Pr∞fWiki",, 2016


  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)