Proposition: 1.30: Transitivity of Parallel Lines
(Proposition 30 from Book 1 of Euclid's “Elements”)
(Straightlines) parallel to the same straight line are also parallel to one another.
Modern Formulation
Straight lines parallel to the same straight line are also parallel to one another, i.e. if \(AB\parallel EF\) and \(EF\parallel CB\) then \(AB\parallel CB\).
Table of Contents
Proofs: 1
Mentioned in:
Proofs: 1 2
Thank you to the contributors under CC BYSA 4.0!
 Github:

 nonGithub:
 @Calahan
 @Casey
 @Fitzpatrick
References
Adapted from CC BYSA 3.0 Sources:
 Callahan, Daniel: "Euclid’s 'Elements' Redux" 2014
Adapted from (Public Domain)
 Casey, John: "The First Six Books of the Elements of Euclid"
Adapted from (subject to copyright, with kind permission)
 Fitzpatrick, Richard: Euclid's "Elements of Geometry"