How to create a transition surface between a tangent line (defined by tangent-continuous surface curves) on one set of surfaces, and an adjoining set of surfaces, along a symmetric tangent line.

1. Choose Surfaces > Multi-Surface Fillet > Symmetric Fillet .
2. Select the surface curves to be used as tangent lines for the fillet. They can be surface edges, trimmed edges, curves-on-surface or isoparms, but they must be tangent continuous (G1).
   

   Start and End labels appear, indicating the extent of the selected curves.

3. Click the Accept button.
4. Select the second set of surfaces.

   The surfaces are drawn in yellow.

5. Click the Accept button.
   

   A blue arrow appears, showing on which side the fillet will be built.

6. Click on the arrow to flip it if needed, then click the Build button.

   The symmetric fillet is built between the tangent line(s) and the second set of surfaces. By default the fillet is circular and extends the full length of the tangent line(s).

   
7. Open the control window and modify the parameters, then click the Recalc button to update the fillet.

   In the example below, Section Type is set to G2 Curvature, Continuity Check is on, and both ends of the fillets have been extended, so that the surfaces can be trimmed (Trim Type is set to Automatic).

   

   The symmetric fillet has construction history and will update whenever the input geometry is modified.