A straight line is at right angles to a plane, when it makes right angles with all the straight lines which meet it and are in the plane.

Εὐθεῖα πρὸς ἐπίπεδον ὀρθή ἐστιν, ὅταν πρὸς πάσας τὰς ἁπτομένας αὐτῆς εὐθείας καὶ οὔσας ἐν τῷ [ ὑποκειμένῳ ] ἐπιπέδῳ ὀρθὰς ποιῇ γωνίας.
