Playfair's axiom is not exactly equivalent to Euclid's Fifth Postulate[7] because on an elliptical geometry, such as the surface of a sphere, Euclid's postulate in its original version:
If a line segment intersects two straight lines forming two interior angles on the same side that sum to less than two right angles, then the two lines, if extended indefinitely, meet on that side on which the angles sum to less than two right angles.