Orient the principal axis of the monument on the rising or setting of the sun at a solstice, so that the architecture frames it.
The henge: the ditch inside the bank
Enclose the gathering place with a circular bank of chalk and dig the ditch on the inside of it, so that the earthwork marks off a sacred interior rather than defending it; leave causeways of undug ground for the entrances.
A fort puts its ditch outside the rampart, to keep enemies out; a henge does the reverse. At Durrington Walls, some 480 m across and dug around 2460 BC, the ditch was about 18 m wide and more than 5 m deep inside a bank about 30 m broad: the largest henge in Britain. Freshly cut chalk is brilliant white, so the bank would have stood out on the green downland for miles.
The rule is inferred from the monuments themselves; no one wrote it down. The inward-facing ditch suggests that the enclosure was meant to contain and to exclude by ritual, not by force, and many henges enclose earlier timber or stone circles.
Related rules
In a megalithic chamber the room can be no wider than the largest slab the builders can raise over it; the upright stones are set to suit the capstone, not the other way round.
Build the circle of the living in timber, which decays, beside the settlement; build the circle of the ancestors in stone, which endures, apart from it; and join the two by avenues and the river.
Run a straight, banked or surfaced avenue from the entrance of each monument down to the river, on the monument’s solstice axis, so that the approach itself is aligned on the sun.
Cite this page Chris Paton, “The henge: the ditch inside the bank”, The Textbook of Architecture, https://textbookofarchitecture.com/rule/henge-inner-ditch. Text, drawings and models licensed under CC BY-NC 4.0.