Proof: By Euclid
(related to Proposition: Prop. 11.09: Lines Parallel to Same Line not in Same Plane are Parallel to each other)
- 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 $CD$.
Adapted from (subject to copyright, with kind permission)
- Fitzpatrick, Richard: Euclid's "Elements of Geometry"