Hi Moi3dFan, just to reiterate what Peer writes above,

re:
> but why can't I make a fillet of a straight segment and a curve?

It's because those segments are not on a single shared plane.


> And a straight line and a straight line form a fillet perfectly.

The 2 straight lines are coplanar so that's why that one works.

Basically, the job of curve fillet is to generate an exact circular arc, which is a planar curve. So it's generally required that all 3 pieces - both input curves and the arc - need to all be on one common plane.

The result you show from solids filleting is not an exact circular arc because it comes from a diagonal cut of the fillet surface.

- Michael