Rules · Cities · Plans · Modules
Divide the capital into blocks (machi) forty jō square; gather sixteen blocks into a ward (bō) and the wards into rows (jō); separate the blocks by lanes of four jō and the wards by avenues of eight to twelve, and make Suzaku-ōji, the central avenue, twenty-eight.
machi = 40 jō (c. 120 m) · kōji 4 jō · ōji 8–12 jō · Nijō 17 jō · Suzaku-ōji 28 jō (c. 84 m) · city 1,508 × 1,753 jō
Heian-kyō, founded in 794, is the last and most regular of the Japanese capitals modelled on Tang Chang’an. Unlike the earlier Heijō-kyō (Nara), whose blocks shrank where the wide avenues ate into them, Heian-kyō kept every block exactly forty jō square and added the streets between them, so that a block was the same size anywhere in the city: the unit in which land was granted, sold and taxed.
The town keeps the true size of the block and the true widths of the avenues and lanes, but has one ward on each side of Suzaku-ōji instead of four, and five rows of blocks south of the palace instead of twenty-eight: about a quarter of the real city in each direction.
Build the bracket set (puzuo) of blocks (dou) and arms (gong) crossing at right angles, each tier projecting no more than 30 fen beyond the one below; a set of n projecting tiers is called an (n + 3)-puzuo, from four to eight.
projection per tier ≤ 30 fen · rise per tier = 21 fen · puzuo = tiers + 3
Give a hall an odd number of bays across the front (three, five, seven, nine or eleven) so that the central axis passes through a bay and a doorway, never through a column.
Lay out the royal city as a square of nine li on each side, with three gates in each wall and, within, nine roads running north–south and nine east–west, each wide enough for nine chariots abreast.
side = 9 li · 3 gates × 4 sides · 9 jing + 9 wei · road width = 9 gui (tracks)
Find true north and south with a plumb-lined post and the shadows of sunrise and sunset, checked by the pole star; then turn the principal hall of every compound to face south.
Fix the bay (the distance between column centres) and derive every other member from it by fixed ratios: the column from the bay, the brackets and beams from the column, the rafters and their spacing from the brackets, the eaves from the rafters.
Set out the posts of a house on a square grid of one ken (six shaku, about 1.82 m) and size rooms, doors, screens and mats to fit the grid.
1 ken = 6 shaku ≈ 1.82 m · 1 tsubo = 1 ken² ≈ 3.3 m² = 2 mats
Cite this page Chris Paton, “The jōbō grid: blocks of forty jō”, The Textbook of Architecture, https://textbookofarchitecture.com/rule/heian-jobo. Text, drawings and models licensed under CC BY-NC 4.0.