Definition: Def. 11.06: Inclination of a Plane to a Plane
The inclination of a plane to a(nother) plane is the acute angle contained by the (straight lines), (one) in each of the planes, drawn at right angles to the common segment (of the planes), at the same point.
Modern Formulation
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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