Source — Euclid's Elements, encoded as an rrxiv paper

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XI.18 Proposition XI.18

If a straight line be at right angles to any plane, all the planes through it will also be at right angles to the same plane.

Proof

Any plane through the perpendicular contains the perpendicular line; by Definition XI.4 (perpendicular planes) the containing plane is perpendicular to the original plane.

lines 74–74 in main.tex