How It works
Duplicating half of the object assures that the object will be perfectly symmetrical when combined. Using the center line allows rough alignment of the halves horizontally, as a continuation. Using the Alignment functions helps make sure the paths are perfectly aligned vertically. Once the paths are aligned both horizontally and vertically, they are combined to produce a perfectly mirrored whole. Storing the shape with the custom shapes makes it available for scalable application via the Shapes tool. Saving the shape with the shape set permanently stores the shape with a library so that it can be recalled if necessary.