Although your cursor appears as a point on your screen, it actually represents a line through space, normal to the screen. In order to be able to pick a point with your cursor, you first need to define an imaginary plane that, when intersected by the normal line of your cursor, will yield a unique point in space. This imaginary plane is called a working plane. Another way to think of the interaction between your cursor and your working plane is to picture your cursor as a point that moves around on your working plane. The working plane, then, acts as a "tablet" on which you write with your cursor. (The working plane need not be parallel to your display screen.)

Figure 4.1: Relationships Among Display Screen, Cursor, Working Plane, and Picked Point

A working plane is an infinite plane with an origin, a 2D coordinate system, a snap increment, and a display grid. You can define only one working plane at a time. (Creating a new working plane eliminates your existing working plane.) The working plane is separate from the coordinate systems. For example, the working plane can have a different point of origin and rotation than the active coordinate system. To learn how to force the active coordinate system to track the working plane, see Working Plane Tracking.