AutoCAD provides two types of coordinate systems. One is a single fixed coordinate system called the World Coordinate System, and the other is an infinite set of user-defined coordinate systems available through the User Coordinate System.

The World Coordinate System (WCS) is fixed and cannot be changed, as indicated in Chapter 2. In this system (when viewing the origin from 0,0,1), the X axis starts at the point 0,0,0, and values increase as the point moves to the operator's right; the Y axis starts at 0,0,0, and values increase as the point moves to the top of the screen; and finally, the Z axis starts at the 0,0,0 point, and values get larger as it comes toward the user. All drawings from previous chapters are created with reference to the WCS. The WCS is still the basic system in virtually all 2D AutoCAD drawings. However, because of the difficulty in calculating 3D points, the WCS is not suited for many 3D applications.

The User Coordinate System (UCS) allows you to change the location and orientation of the X, Y, and Z axes to reduce the number of calculations needed to create 3D objects. The ucs command lets you redefine the origin of your drawing and establish the positive X and the positive Y axes. New users think of a coordinate system simply as the direction of positive X and positive Y. But once the directions of X