Make a curve symmetrical with the Object Edit > Symmetric Modeling tool, then modify both sides simultaneously.

The following workflow assumes that a curve has been created across the Y axis.

To modify a curve across a centerline

1. Choose Object Edit > Symmetric Modeling.
2. Select the curve.

   If the curve is not already symmetric, the tool modifies the controls to make it symmetric. The position of the controls from the beginning of the curve are applied to the symmetric partners at the end of the curve. Click the Flip Master Side button to have it work the other way around.

   Note

   Several curves (and surfaces) can be selected at once. Symmetry constraints are applied to each curve (or surface) individually.

   A yellow symmetry plane is displayed. This plane corresponds to the default symmetry plane for the layer the object belongs to (XZ plane by default). You can modify it by using Layers > Symmetry > Set Plane.

   
3. Pick one or more CVs on one side of the curve.

   Blue circles appear around the corresponding CVs on the symmetric half.

4. Move the CVs. You can use any transformation tool, such as Transform > Move or Control Panel > Transform CV.
   
   • The selected CVs follow the mouse. The symmetric CVs move in the opposite direction on the symmetric side of the plane.
     Note

     To have the symmetric CVs update as you drag the mouse, turn on During Transform in the Construction History Updates section of Preferences > Performance Options.

   • The curve has construction history so that any subsequent operation on the CVs will continue to maintain symmetry across the plane. If you want to remove the symmetry constraint from the geometry, delete the construction history (Delete > Delete Construction History).
   • If you move the curve, the symmetry plane moves with it but is drawn in a different color to differentiate the curve from symmetric objects still aligned to the layer symmetry plane. A confirm box warns you. Clicking Cancel undoes the transformation.