• reflexivity: a = a,
• symmetry: if a = b then b = a,
• transitivity: if a = b and b = c then a = c, where c is a third entity,
• if a = b and ℛ{u} is any statement involving the entity u, then ℛ{a} is true if and only if ℛ{b} is true

NOTE The equality of two entities a and b is denoted by a = b and expressed by "a is equal to b".