The Symmetry tool
enables you to create symmetrical copies of entities
on either side of a symmetry line. The two entities are mutually
constrained. The Symmetry
tool is used together with a selected drawing tool.
To create symmetrical entities:
Select a geometry creation tool.
Select the Symmetry
tool .
The cursor appears in the graphical display
area as a crosshair-and-symmetry symbol 
.
Set a symmetry line by clicking two points in the graphical display area (or select an existing Symmetry line).
The Symmetry tool is now active. The cursor symbol now reflects the drawing tool that was selected in step 1. In the example below, the Circle tool was selected. You can now create symmetrical circles.
Create entities on one side of the symmetry line, and click.
Their mirror image is immediately reflected on other side of the symmetry line.
Click the Symmetry
tool again (to de-select the Symmetry
tool), or click Select 
, to end the process. (Clicking exit does
not end the symmetry process.)
Tip: If you move the symmetry line using the Select tool, all the entities that you created while the symmetry process was active (while the Symmetry tool was selected) will be constrained around the symmetry line. In the example below, the symmetry line is moved to the left, causing the images around it to converge.
Notes:
Symmetry lines also appear when mirroring entities. These symmetry lines can be used after the mirroring operation.
When using symmetry lines during regular dimensioning operations, it is important to note that the position on the symmetry line that you pick, could influence the resulting dimension.