The Mirror tool allows you to create a mirror image of any user defined nodes, members or plates about any user defined surface.
After selecting the nodes, members or plates to be mirrored, right-click and select "Mirror" from the menu that appears. You should then pick a point that lies anywhere in the mirror plane followed by filling out the form shown below.
Don't forget that when picking points in the renderer, you can attach to other nodes or members, or you can "lock on" to a node or member and then align with an orthogonal line or an extension line from the "locked on" node or member. For more information, refer to Attachment and alignment methods. Remember also that when drawing, you can use the mouse or you can simply type in the coordinates of the desired point(s). For more information, refer to "Positioning points using the keyboard".