@gaieus said:

What exactly is a "stringer" in this case?

On a straight stair, the stringer is the member on each side of the stair that the stair tread attaches to. In this case the stringer is the helix shape on each side of the spiral stair that encloses or bounds each stair stringer.

I'm new to SketchUp and have version 8. Is there a way to create a continuous stringer for a sprial stair?
Thx.