Dog-leg is a term used to describe a configuration of stairs between two floors of a building, often a domestic building, in which there are two short flights at 180 degrees to each other, joined by a half-landing to enable the 180 degree turn. The flights do not have to be equal, and frequently are not.
Structurally the flights of a dog-leg stair are usually supported by the half-landing, which spans the adjoining flank walls.
From the design point of view the main advantages of a dog-leg stair are:
To allow an arrangement that occupies a shorter, though wider, floor area than a straight flight, and so is more compact. Even though the landings consume total floor space, there is no large single dimension
The upper floor is not directly visible from the bottom of the stairs, thereby providing more privacy