Rules · Stairs · Proportion
Proportion every step so that twice its height plus its depth equals the length of a natural pace on the level: two feet, or 24 pouces.
2 r + g = 24 pouces ≈ 65 cm (modern practice: 60–66 cm)
Blondel observed that a person walking on the level takes a step of about two French feet, and reasoned that raising the foot costs about twice as much effort as carrying it forward. A step that is higher must therefore be shallower, and one that is lower must be deeper, so that the sum 2r + g stays equal to the natural stride. With a riser of 6 pouces (16 cm) the going is 12 pouces (32 cm); with a riser of 5 pouces the going becomes 14.
The rule is still written, in one form or another, into building regulations across Europe, usually as 2R + G between about 550 and 700 mm with limits on the riser. Try the controls: a riser of 20 cm with a going of 22 cm still satisfies the sum but is too steep to be comfortable, which is why codes also cap the riser and the pitch.
To find how thick the pier of an arch must be, divide the intrados into three equal parts; draw a line from the springing through the first division, and produce it beyond the springing by its own length. A vertical through the end of that line marks the outer face of the pier.
pier ≈ ¼ span (semicircle) · ≈ 0.23 span (equilateral) · less for a lancet
To unify a façade of two or more storeys, run a single order of pilasters or columns through all of them, from the base to the main cornice.
Hold that the beauty of the orders comes from fixed proportions that please by nature, as consonances do in music; where the ancients differ, the differences answer to circumstances such as distance and height.
Make the stair of a palace a processional space: one broad flight rising to a landing, then turning back in two flights either side to arrive at the principal floor beneath a single great vault.
Give the flight of steps before a temple an odd number, so that one who starts with the right foot will also set the right foot first on the top.
Make the parterre oblong rather than square, since figures somewhat oblong look better on the ground than square ones; later practice made it as broad as the front of the house and about twice as long.
parterre oblong; later practice: width ≈ house front, length ≈ 2 × width
Cite this page Chris Paton, “Two risers and a going make a pace”, The Textbook of Architecture, https://textbookofarchitecture.com/rule/blondel-stair-rule. Text, drawings and models licensed under CC BY-NC 4.0.