Hilbert's axioms are a set of 20 assumptions proposed by David Hilbert in 1899 in his book Grundlagen der Geometrie (tr. The Foundations of Geometry) as the foundation for a modern treatment of Euclidean geometry. Other well-known modern axiomatizations of Euclidean geometry are those of Alfred Tarski and of George Birkhoff. The axioms Hilbert's axiom system is constructed with six primitive notions: three primitive terms: point; line; plane; and three primitive relations: Betweenness, a ternary relation linking points; Lies on (Containment), three binary relations, one linking points and straight lines, one linking points and planes, and one linking straight lines and planes; Congruence, two binary relations, one linking line segments and one linking angles, each denoted by an infix ≅. Line segments, angles, and triangles may each be defined in terms of points and straight lines, using the relations of betweenness and containment. All points, straight lines, and planes in the following axioms are distinct unless otherwise stated. I. Incidence For every two points A and B there exists a line a that contains them both. We write AB = a or BA = a. Instead of "contains", we may also employ other forms of expression; for example, we may say "A lies upon a", "A is a point of a", "a goes through A and through B", "a joins A to B", etc. If A lies upon a and at the same time upon another line b, we make use also of the expression: "The lines a and b have the point A in common", etc. For every two points there exists no more than one line that contains them both; consequently, if and , where , then also . There exist at least two points on a line. There exist at least three points that do not lie on the same line. For every three points A, B, C not situated on the same line there exists a plane α that contains all of them. For every plane there exists a point which lies on it. We write . We employ also the expressions: "A, B, C lie in α"; "A, B, C are points of α", etc. For every three points A, B, C which do not lie in the same line, there exists no more than one plane that contains them all. If two points A, B of a line a lie in a plane α,