To combine the unity of a central plan with the direction a church needs, plan the space as an oval, set across the axis to widen the view of the altar, or along it to lead the eye towards it.
Rules · Cities · Plans · Optics
Three streets from one gate
Where the main road enters the city, open a piazza inside the gate and send three straight streets from it into the town, fanning from a common point marked by an obelisk, with a pair of matching churches in the angles between them.
The Roman trident grew over a century. The Corso follows the ancient Via Flaminia; the Via di Ripetta was cut under Leo X, and the Via del Babuino in the following decades. Sixtus V set the Egyptian obelisk at their meeting point in 1589, and Carlo Rainaldi, with Bernini and Carlo Fontana, built the two churches of Santa Maria dei Miracoli and Santa Maria in Montesanto to frame the Corso. They look like twins, though the sites differ: one church has a round dome and the other an oval one, set so that both read alike from the piazza.
A visitor arriving from the north saw three long perspectives open at once. Versailles took the same figure into the landscape as the patte d’oie, the goose-foot of avenues before the palace.
Related rules
Divide the inside of a dome into coffers that shrink in step with the meridians as they rise, so that their perspective deepens the dome and draws the eye to the light of the lantern.
Let the wall of a façade curve in plan (concave, convex, concave) so that the front answers the street, the light and the space within, and the order bends with it.
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.
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
Proportion the width of an allée to its length: a walk of about 100 toises (some 200 m) should be 5 or 6 toises wide, one of 200 toises 7 or 8, and so on, so that a long walk does not seem to narrow to a slot in the distance.
100 toises long → 5–6 wide · 200 → 7–8 · 300 → 9–10 · 400 → 10–12 (1 toise ≈ 1.95 m)
Cite this page Chris Paton, “Three streets from one gate”, The Textbook of Architecture, https://textbookofarchitecture.com/rule/trident-piazza. Text, drawings and models licensed under CC BY-NC 4.0.