For any permutation group, the action that sends ("g", "x") → "g"("x") is called the natural action of "G" on "M". This is the action that is assumed unless otherwise indicated. In the example of the symmetry group of the square, the group's action on the set of vertices is the natural action. However, this group also induces an action on the set of four triangles in the square, which are: "t"1 = 234, "t"2 = 134, "t"3 = 124 and "t"4 = 123. It also acts on the two diagonals: "d"1 = 13 and "d"2 = 24.