The XForm>Mirror functions can be used to reflect selected geometry across a plane. To access the Mirror>Move, Copy, and Join functions, select Mirror from the XForm Menu.
Using the Function:
Select Mirror from the Transform Menu.
Select Move, Copy, or Join (see icons below) from the Transform>Mirror submenu.
Mirror>Move
Mirror>Copy
Mirror>Join
Select the entities you intend to mirror.
Define the mirroring plane by using one of the following options:
Mirror>Move
Mirror>Copy
Mirror>Join