## ABSTRACT

“Real” objects are three-dimensional, or 3D. So why should we consider 2D objects, such as the 2D lines in this chapter? Because they really are the building blocks for geometric constructions and play a key role in many applications. We look at various representations for lines, where each is suited for particular applications. These line forms include the parametric, explicit, and implicit forms. The normal to a line is defined and then we introduce the point-normal form of a line. Once we can represent a line, we can perform intersections and determine distances from a line. The foot of a point is introduced. A closest point of approach application applies the ideas introduced in this chapter. Sketches and figures illustrate the concepts.