In general usage, symmetry most often refers to mirror or reflective symmetry; that is, a line (in 2-D) or plane (in 3-D) can be drawn through an object such that the two halves are mirror images of each other. An isosceles triangle and a human face are examples. Line symmetry is when there exists a line (or lines) such that the image of a figure when reflected over the line is itself. A nontrivial rotational symmetry of a figure is a rotation of the plane that maps the figure back to itself such that the rotation is greater than 0° but less than 360°. Reflection symmetry, line symmetry, mirror symmetry, mirror-image symmetry, is a symmetry to reflection. That is, a figure which does not change upon undergoing a reflection has reflectional symmetry. Rotational symmetry, also known as radial symmetry in biology, is the property a shape has when it looks the same after some rotation by a partial turn. An object's degree of rotational symmetry is the number of distinct orientations in which it looks the same for each rotation. Chapter 17 discusses the lines of symmetry of given geometrical figures, its reflection and rotation.