When drawing in 3D, you specify X, Y, and Z coordinate values in either the World Coordinate System (WCS) or the current user coordinate system (UCS). The illustration below shows the X, Y, and Z axes of the WCS.
To enter coordinates relative to the WCS, precede coordinate values with an asterisk (*). For example, entering @*2,0,0 specifies a point two units to the right (the X direction) of the last point entered relative to the WCS; entering @2,0,0 specifies a point two units in the X direction of the last point entered relative to the UCS. In practice, most coordinates are entered relative to the UCS rather than the WCS.
The WCS and the UCS are often coincident – their axes and origin points overlap exactly. No matter how you reorient the UCS, you can always make it coincident with the WCS by using the World option of the UCS command.
Use the right-hand rule to determine the positive axis direction of the Z axis when you know the direction of the X and Y axes in a 3D coordinate system. You can also use the right-hand rule for determining the positive direction of rotation about an axis in 3D space.
To determine the positive axis direction of the X, Y, and Z axes in the UCS, place the back of your right hand near the screen. Point your thumb in the direction of the positive X axis. Extend your index and middle fingers as illustrated, and point your index finger in the direction of the positive Y axis. Your middle finger indicates the direction of the positive Z axis. By rotating your hand, you get a general idea of how the X, Y, and Z axes rotate as you change the UCS.
To determine the positive rotation direction about an axis, point your right thumb in the positive direction of the axis and curl your fingers as illustrated. Your fingers indicate the positive rotation direction about the axis.