DESCRIPTION
You can enable Local Symmetry (was previously Rhombus) to create symmetric Bezier Polygon/2D/3D Polylines.
(Symmetry Y can also be applied to Triangle and Joint shapes to create Rhombus and Football shapes).
(Symmetry Z can also be applied to Extruded shapes with cone properties to create two-sided cone/beveled shapes).
The first and last control points will be fixed on the axes. You can also change their continuity to C1 to make the shape boundary smooth across the axes.
For Polylines, the control points are limited in the positive regions.
You can enable "Display Mirror Plane" (or CTRL+M) to display the mirror axes. When you select one of the control points, the mirror plane will switch to the local mirror axes from the global mirror axes.