2004

226 | Chapter 15 Use Precision Tools
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.
Apply the Right-Hand Rule
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.
Place the back of your right hand near the screen and point your thumb in
the direction of the positive X axis. Extend your index and middle fingers as
illustrated, pointing 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 see how the X, Y, and Z axes rotate as you change the UCS.
You can also use the right-hand rule for determining the positive direction of
rotation about an axis in 3D space. Point your right thumb in the positive
direction of the axis and curl your fingers. Your fingers indicate the positive
rotation direction about the axis.
To enter coordinates relative to the WCS
■ Precede coordinate values with an asterisk (*).
Entering @*2,0,0 specifies a point two units in 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.
+Y
+X
+Z
-Z
-Y
WCS origin point
-X
Z
Y
X