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Symmetric Constraint

In one sentence: Force two points (or two edges) to be mirror images of each other about a symmetry axis — a line, a point, or one of the sketch axes.

Overview

Workbench: Sketcher · Menu: Sketch → Sketcher constraints → Symmetric Constraint
Toolbar: Sketcher Constraints · Shortcut: S

Symmetric removes 1 DoF by constraining the midpoint of the line joining two selected points to lie on the axis, and the joining line to be perpendicular to the axis. This ensures perfect bilateral symmetry.

Intuition

Think of folding a piece of paper along a crease line: whatever is on the left side is exactly mirrored on the right. Symmetric encodes that mirror relationship as a constraint — you can move either point or the axis, and the symmetry is maintained automatically.

When to use it

  • You have a symmetric profile and want to draw only one half and let the other half follow automatically.
  • You need the centre of a feature (e.g. a slot) to be symmetric about the sketch Y-axis.
  • You are positioning two identical holes equidistant from the centre.
  • Ensuring a rectangular profile is centred on the origin (apply Symmetric to opposite corners with the origin as the axis point).

When NOT to use it

  • You want a mirrored copy as independent geometry — use Mirror Sketch or Symmetry (transform) instead.
  • The symmetry axis should be a new line — draw the line first, then apply Symmetric.

Step-by-step walkthrough

Symmetric about a line

  1. Select the first point, the second point, and the symmetry line (Ctrl-click each in sequence, line last).
  2. Press S or activate Sketch → Symmetric Constraint.
  3. The two points move to be symmetric about the line.

Symmetric about a point

  1. Select the first point, the second point, and the symmetry point (midpoint).
  2. Press S. The two outer points are mirrored through the centre point.

Tip

To make a sketch symmetric about the sketch Y-axis, select the two points and then select the Y-axis line (the vertical construction line through the origin).

Parameters

No numeric parameters. Removes 1 DoF.

Common mistakes and pitfalls

Three elements must be selected in the correct order

Cause: Symmetric requires exactly 3 selections: point 1, point 2, and the axis (line or point). Selecting in the wrong order or selecting only 2 elements will not apply the constraint.
Fix: Deselect all, then click point 1, Ctrl-click point 2, Ctrl-click the axis.

Python API

Minimal example

import FreeCAD as App
import Sketcher
import Part

doc = App.activeDocument()
sketch = doc.getObject("Sketch")

# Two points symmetric about the Y-axis (geoId -2 = Y-axis)
pt0 = sketch.addGeometry(Part.Point(App.Vector(-10, 5, 0)), False)
pt1 = sketch.addGeometry(Part.Point(App.Vector(10, 5, 0)), False)

# Symmetric about the sketch Y-axis
sketch.addConstraint(Sketcher.Constraint("Symmetric", pt0, 1, pt1, 1, -2, 1))
doc.recompute()

Method signature

sketch.addConstraint(Sketcher.Constraint(
    "Symmetric",
    geo1,    # int — first element
    vertex1, # int — vertex on first element (1=start, 2=end, 3=centre)
    geo2,    # int — second element
    vertex2, # int — vertex on second element
    geoSym,  # int — symmetry axis geometry id (-2=Y-axis, -3=X-axis, or a line)
    vertexSym, # int — vertex on symmetry axis (3 = midpoint/any point for a line)
))

See also