I found a way to pull a shape down from a line and lost it

Thanks! I'll explore your suggestion, Eskimo, but what I was trying to remember was pretty easy:

1. Create a two-point line
2. Convert the line to a curve.
3. Select one or both nodes with the shape tool and click Close Curve (oddly, this reverses the direction of the line segment)
4. There is now a (straight) curve superimposed on the original line. Use the shape tool to pull the curve outward.

You'll get a shape like this:

Useful, for instance, if you initially set the straight line along a vertical guide, pull out the curve as above, then mirror the new shape on the other side of the guide. Quick route to a symmetrical shape.