- Choose the degree of the spline in the options tool bar. Supported
    degrees are 2 (quadratic b-spline) and 3 (cubic b-spline). The higher the
    degree, the 'smoother' the curve becomes. 
    
 Note that a quadratic b-spline requires by definition at least 3
    control points while a cubic b-spline has at least 4 control points.
- Check the 'Closed' check box in the options tool bar if you want to
    create a closed spline. Closed splines are continuous closed loops.
- Specify the control points. After defining the third (for quadratic
    b-splines) or fourth (for cubic b-splines) control point, a preview will be
    shown. You can remove the last control point again by clicking the 'Undo'
    button in the options tool bar.
- Hit escape or click the right mouse button after setting the last point
    to confirm the spline shape.
- You can not create another spline or right-click again to terminate the
    tool.
Example for two open splines (quadratic and cubic): 
  
   
  
Example for two closed, periodic splines (quadratic and cubic): 
  
  