|Straight lines parallel to the same straight line are also parallel to one another.|
|Let each of the straight lines AB and CD be parallel to EF.
I say that AB is also parallel to CD.
|Let the straight line GK fall upon them. Since the straight line GK falls on the parallel straight lines AB and EF, therefore the angle AGK equals the angle GHF.||I.29|
|Again, since the straight line GK falls on the parallel straight lines EF and CD, therefore the angle GHF equals the angle GKD.||I.29|
|But the angle AGK was also proved equal to the angle GHF. Therefore the angle AGK also equals the angle GKD, and they are alternate.||C.N.1|
|Therefore AB is parallel to CD.|
|Therefore straight lines parallel to the same straight line are also parallel to one another.|
In many modern expositions of synthetic geometry, Playfair's axiom (John Playfair, 1748-1819) is chosen as that postulate instead of Euclid's parallel postulate Post.5. Playfair's axiom states that there is at most one line parallel to a given line passing through a given point. (That there is at least one follows from the next proposition I.31 which doesn't depend on the parallel postulate.)
Two advantages of Playfair's axiom over Euclid's parallel postulate are that it is a simpler statement, and it emphasizes the distinction between Euclidean and hyperbolic geometry.
Two disadvantages are that it does not have the historical importance of Euclid's parallel postulate, and the proof of the parallel postulate from Playfair's axiom is nonconstructive. That proof is a proof by contradiction that begins assuming that a point does not exist, deriving a contradiction, and concluding that the point must exist, but does not construct it. It may well be that Euclid chose to make the construction an assumption of his parallel postulate rather rather than choosing some other equivalent statement for his postulate.
One of the criticisms of Euclid's parallel postulate was that it isn't simple. The statement of this proposition, I.30, is much simpler, and Playfair's axiom is much simpler. As they're each logically equivalent to Euclid's parallel postulate, if elegance were the primary goal, then Euclid would have chosen one of them in place of his postulate. Perhaps the reasons mentioned above explain why Euclid used Post.5 instead.
Next proposition: I.31
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