This staircase, in Abel’s own house, replaced what most people would consider a ladder. It is a “winder”: the treads wind around a center post.
Modern codes set a minimum width at the narrow end of a winding tread, so the days of winding around a single point are gone. For geometry fans, this moves the center of the layout to a point some distance away from the stair. The inside stringer is mitered and the outside stringer is housed, with traditional wedging on the housed side. The varying stringer angles inspired the curved handrail.
