I needed to make a hose connector with a screw thread, see image.
But the hose screwing into the connector has a left-handed thread.
the obvious way to make a thread is to sweep a spline along a helix. There is no way I could find to make a left-handed helix!
I solved the problem by using a right-handed helix, using a mirror to get a left-handed one, then converting the prinitives to meshes with "current state to object", then deleting the right-handed thread.

Why is there no way to choose the handedness of the helix?