The Birail operator lets you extrude an open-profile curve by running its endpoints along two rails, or guide curves. This is sometimes called guided extrusion. The distance between points determines which end of which rail is connected to each end of the profile.
Select the profile curve, then add the two rail curves to the selection. You can select curve objects as well as U or V isolines, U or V knot curves, boundaries, surface curves, and trim curves.
Choose one of the following commands from the Model toolbar:
If you didn't select all the curves in step 1, you can pick the remaining ones now in order. This creates an object and the Birail property editor opens.
Adjust the parameters as desired. For more information, see Birail Property Editor [Properties Reference].
Birail showing the profile curve (A), the two rail curves (B and C), and the result.
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