Axiom: 1.2: Segment Extension
(Postulate 2 from Book 1 of Euclid's “Elements”)
And to produce a finite straight line continuously in a straight line.
Modern Formulation
A segment may be extended from any length to a longer segment, a ray, or a straight line.
Mentioned in:
Parts: 1
Proofs: 2
Propositions: 3
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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"