Lines are also classified by their relationship to other angles and lines. *Perpendicular lines* are two lines, segments, or rays that intersect to form a right (90 degree) angle. *Orthogonal lines* are another way of saying lines are perpendicular—but it is mostly used in terms of functions, transformations, and vectors in other mathematical fields. *Normal lines* are perpendicular lines where each line is perpendicular to a curve (including a line) or a surface (including a plane). And finally, a line is considered to be *tangent* to a circle if it intersects the circle at exactly one point—also called the *point of tangency.* Such lines are also called tangential lines.