(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 $CD$. <prf>  Thus, $AB$ is parallel to $CD$ [Prop.~1.27]. Thus, (straight-lines) parallel to the same straight-line are also parallel to one another.