Use a symmetry plane to force the generated shape to be symmetric about the plane. Up to three symmetry planes can be used on the model, one for each of the local XY, XZ, and YZ planes. The symmetry plane can either reference an existing UCS or be aligned with the global coordinate system and intersect the center of mass of the part.
Symmetry Plane. By default, the symmetry plane is positioned at the center of mass of the part and aligned with the global coordinate system.
After you have created a symmetry plane, you can edit, suppress, or delete the plane by right-clicking the symmetry plane in the browser tree.