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