Land and Zoning. A line on a map is a price.

Zoning rules look purely technical: heights, setbacks, floor area ratios. They are also the strongest lever anyone has on what housing costs and who ends up living next to whom. Where the rules cap how much can be built, prices rise, and the increase falls hardest on the people with the least.

• A city that will not build upward or outward has chosen higher rents. Land is fixed, but the floor space allowed on top of it is a decision somebody made.
• In the United States, zoning and lending rules were used deliberately to segregate neighbourhoods. Maps drawn in the 1930s still show up in today's wealth gaps.