Autodesk AUTOCAD 2006, Command Reference Manual, Page: 1248 / 1490

Define a UCS by selecting an object
Method of determining UCS Object
The point becomes the new UCS origin. Point
The start point of the polyline is the new UCS origin. The X axis
extends along the line segment from the start point to the next
vertex. 2D polyline
The first point of the solid determines the new UCS origin. The new
X axis lies along the line between the first two points. Solid
The “from” point of the trace becomes the UCS origin, with the X
axis lying along its centerline. Trace
The new UCS origin is taken from the first point, the X axis from the
first two points, and the Y positive side from the first and fourth
points. The Z axis follows by application of the right-hand rule. 3D face
The insertion point of the object becomes the new UCS origin, and
the new X axis is defined by the rotation of the object about its Shape, text, block
reference, attribute
definition
extrusion direction. The object you select to establish a new UCS has
a rotation angle of zero in the new UCS.
Face
Aligns the UCS to the selected face of a solid object. To select a face, click
within the boundary of the face or on the edge of the face. The face is
highlighted and the UCS X axis is aligned with the closest edge of the first
face found.
Select face of solid object:
Enter an option [Next/Xflip/Yflip] <accept>:
Next
Locates the UCS on either the adjacent face or the back face of the selected
edge.
Xflip
Rotates the UCS 180 degrees around the X axis.
Yflip
Rotates the UCS 180 degrees around the Y axis.
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