Proposition: 3.12: Line Joining Centers of Two Circles Touching Externally
(Proposition 12 from Book 3 of Euclid's “Elements”)
If two circles touch one another externally then the (straight line) joining their centers will go through the point of union.
Modern Formulation
If two circles touch one another externally at a point $A$, then the straight line going through their centers goes also through $A$.
Table of Contents
Proofs: 1
Mentioned in:
Sections: 1
Thank you to the contributors under CC BYSA 4.0!
 Github:

 nonGithub:
 @Fitzpatrick
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