Proposition 30
Theorem
If two lines (AB , CD ) are parallel to the same line (EF ), then they are parallel to one another.
Commentary
Original statement
αἱ τῇ αὐτῇ ϵὐθϵίᾳ παράλληλοι καὶ ἀλλήλαις ϵἰσὶ παράλληλοι.
English translation
Straight lines parallel to the same straight line are also parallel to one another.