A leaning structure does not stand over its own base. This measure is the horizontal distance by which it reaches past the perimeter at ground level, taken at right angles to that perimeter.

It describes the lean, not the height. Two towers of equal height can lean by very different amounts, and this figure is what makes them comparable.

The reading is taken where the overhang is greatest. A deliberate lean, such as a designed cantilever, is measured the same way, so the figure describes geometry and says nothing about whether a building is sound.