Use the Mirror command to copy selected entities onto the opposite side of a specified mirror line. To define the mirror axis, specify two points.
To mirror drawing entities:
To force the mirror line to be parallel to the X- or the Y-axis, enable Ortho mode.
The entities are mirrored about the specified axis.

Access
Command: Mirror
Menu: Modify > Mirror
Tool Palettes: Draw > Mirror
Ribbon: Home > Modify > (Flyout) Mirror