The tread depth shall be not less than 10 inches (254 mm). The tread depth shall be measured horizontally between the vertical planes of the foremost projection of adjacent treads PHOTO 17 and at a right angle to the tread’s leading edge. The greatest tread depth within any flight DRAWING 12 (p. 8) of stairs shall not exceed the smallest by more than ⅜ inch (9.5 mm) PHOTO 18.