According to NFPA 101, Life Safety Code, the tread depth is the horizontal dimension of a stair step. It is measured from the leading edge of one tread to the leading edge of the adjacent tread, or to the vertical plane of the riser above. The diagram shows dimension 'B' representing this horizontal measurement, which is the surface a person steps on. This dimension is fundamental in stair design for ensuring safe foot placement during ascent or descent.

A: This dimension represents the riser height, which is the vertical distance between the surfaces of two consecutive treads.

C: This dimension represents the nosing, the portion of the tread that projects over the riser below it.

D: This is a combination of the riser height and tread depth and does not represent a single, standard stair component dimension.

1. NFPA 101®, Life Safety Code®, 2021 Edition. Section 3.3.258.7, Definition of Stair Tread Depth: "The horizontal projected distance from the leading edge of a stair tread to the leading edge of the adjacent stair tread."

2. NFPA 101®, Life Safety Code® Handbook, 2021 Edition. Exhibit 7.23, which accompanies Section 7.2.2.2.1, visually defines the components of a stair, clearly labeling the horizontal dimension (equivalent to 'B' in the question) as "Tread depth."

3. SFPE, The SFPE Handbook of Fire Protection Engineering, 5th Edition. Chapter 55, "Means of Egress," Section 55.4.2.2, Stairs. This section discusses stair geometry, where tread depth (run) is defined as the horizontal component of the step. (DOI: https://doi.org/10.1007/978-1-4939-2565-055)