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.