The rise height of each step is measured from the top of one tread to the next. It is not the physical height of the riser; the latter excludes the thickness of the tread. The tread depth or length is measured from the edge of the nosing to the vertical riser. It is sometimes called the going. The total run of the stairs is the horizontal distance from the first riser to the last riser. It is often not simply the sum of the individual tread lengths due to the nosing overlapping between treads. The total rise of the stairs is the height between floors (or landings) that the flight of stairs is spanning. The slope of the stairs is the total rise divided by the total run (not the individual riser and treads due to the nosing). It is sometimes called the rake or pitch of the stairs. The pitch line is the imaginary line along the tip of the nosing of the treads. Headroom is the height above the nosing of a tread to the ceiling above it. Walkline - For curved stairs, the inner radius of the curve may result in very narrow treads. The "walkline" is the imaginary line some distance away from the inner edge on which people are expected to walk. Building code will specify the distance. Building codes will then specify the minimum tread size at the walkline. To avoid confusion, the number of steps in a set of stairs is always the number of risers, not the number of treads.