Nosing
An edge part of the tread that protrudes over the riser beneath. If it is present, this means that horizontally, the total "run" length of the stairs is not simply the sum of the tread lengths, the treads actually overlap each other slightly.
Riser
The vertical portion between each tread on the stair. This may be missing for an "open" stair effect.
Tread
The part of the stairway that is stepped on. It is constructed to the same specifications (thickness) as any other flooring. The tread "width" is measured from the outer edge of the step to the vertical "riser" between steps.
