These symmetries are functions. Each sends a point in the square to the corresponding point under the symmetry. For example, r 1 sends a point to its rotation 90° clockwise around the square's center, and f h sends a point to its reflection across the square's vertical middle line. Composing two of these symmetries gives another symmetry. These symmetries determine a group called the dihedral group of degree four, denoted ⁠ D 4 ⁠. The underlying set of the group is the above set of symmetries, and the group operation is function composition. Two symmetries are combined by composing them as functions, that is, applying the first one to the square, and the second one to the result of the first application. The result of performing first a and then b is written symbolically from right to left as b ∘ ∘ a ("apply the symmetry b after performing the symmetry ⁠ a ⁠ "). This is the usual notation for composition of functions.