With the Mirror X command, the object's symmetry with respect to the X axis is taken at the drawing stage. Mirror X command is used when drawing doors/windows, columns, strips and retaining walls.
Location of Mirror X Command
You can access it from the ODT door/window toolbar.
You can access it from the column toolbar.
You can access it from the strip footing toolbar.
You can access it from the retaining wall toolbar.
Column before mirror X
Column after mirror X
Door before mirror X
Door after mirror X