Use Grip Modes
 
 
 

To mirror objects using grips

  1. Select the objects to mirror.
  2. Select a base grip on an object by clicking the grip.

    The selected grip is highlighted, and the default grip mode, Stretch, is active.

  3. Cycle through the grip modes by pressing ENTER until the grip mode Mirror appears.

    Alternatively, you can right-click to display shortcut menu modes and options.

  4. Click to specify the second point of the mirror line.

    Turning on Ortho mode is often useful when mirroring objects.