The rounded edge of a stair tread that projects over the riser.
A stair tread is the horizontal portion of a set of stairs on which a person walks.
Risers are the vertical pieces on the stairs. The height of each step is called the rise.
Skirt boards, also called stringers, are the lowest part of an interior wall along a stair case. Skirt boards provide a decorative element to your stairs while covering any gap between the stairs and the wall.