Definition: Def. 11.04: Plane at Right Angles to a Plane
A plane is at right angles to a(nother) plane when (all of) the straight lines drawn in one of the planes, at right angles to the common section of the planes, are at right angles to the remaining plane.
Modern Formulation
(not yet contributed)
Mentioned in:
Proofs: 1 2 3
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References
Adapted from (subject to copyright, with kind permission)
 Fitzpatrick, Richard: Euclid's "Elements of Geometry"
Adapted from CC BYSA 3.0 Sources:
 Prime.mover and others: "Pr∞fWiki", https://proofwiki.org/wiki/Main_Page, 2016