Proposition: Prop. 11.09: Lines Parallel to Same Line not in Same Plane are Parallel to each other
(Proposition 9 from Book 11 of Euclid's “Elements”)
(Straightlines) parallel to the same straight line, and which are not in the same plane as it, are also parallel to one another.
* For let $AB$ and $CD$ each be parallel to $EF$, not being in the same plane as it.
* I say that $AB$ is parallel to $CD$.
Modern Formulation
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Table of Contents
Proofs: 1
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