Starting from two primitive relations whose fields are a dense universe of points, Tarski built a geometry of line segments. According to Tarski and Givant (1999: 192-93), none of the above axioms are fundamentally new. The first four axioms establish some elementary properties of the two primitive relations. For instance, Reflexivity and Transitivity of Congruence establish that congruence is an equivalence relation over line segments. The Identity of Congruence and of Betweenness govern the trivial case when those relations are applied to nondistinct points. The theorem "xy"≡"zz" ↔ "x"="y" ↔ "Bxyx" extends these Identity axioms.