From FreeCAD Documentation
- Select one or more edges and/or points.
- There are several ways to invoke the tool:
- Press the Symmetry button.
- Select the Sketch → Sketcher tools → Symmetry option from the menu.
- The keyboard shortcut: Z then S.
- The cursor changes to a cross with the tool icon.
- The Symmetry parameters section is added at the top of the Sketcher Dialog.
- Optionally press the U key or check the Delete original geometries checkbox to only keep the mirrored elements.
- Optionally press the J key or check the Create Symmetry constraints checkbox to add a Symmetric constraint between each original and mirrored point.
- Select a line or a sketch axis to define the symmetry line, or a point to define the symmetry point.
- Mirrored copies are created. Constraints restricted to the selected elements are also processed
- If Create Symmetry constraints has been selected Symmetric constraints are added.
- If Delete original geometries has been selected the original geometry is removed.