Playfair’s Axiom. Through a given point, not on a given line, exactly one line can be
drawn parallel to the given line.
Playfair’s Axiom is equivalent to the Fifth Postulate in the sense that it can be deduced from
Euclid’s five postulates and common notions, while, conversely, the Fifth Postulate can deduced
from Playfair’s Axiom together with the common notions and first four postulates.