Stairway Walks Line
The stairway walks line is usually located 12 inches from
the inside of the stairway. Most of the stair tread minimum measurements
are taken from these points on circular, winder and spiral stairways.
If you could imagine walking up a set of stairs, holding on to the handrailing, there is a good chance that you're going to be about 12 inches from the interior handrailing of the stairway (providing that you're holding on to the interior handrailing and not the exterior handrailing of the stairway).
If you look at a stairway that's been used for more than five years, you will notice that most of the wear and tear on the stair treads is located about 12 inches from each side, in the center of the stairway.