Boundaries - Step 1

1. Create a new blend with the three splines and select the left curve as the first boundary.
   

2. Now, try to select the right curve as the second boundary. This curve is a composite curve but in this situation only its individual curves can be selected.
   
   The warning message shown herehere is displayed.

   

   

3. Exit the Blend function, click the Composite Curve buttonComposite Curve button, and make this curve into a spline as seen below.