You can walk across Monaco. Not around it, across it, from the French border on one side to the French border on the other, and it takes about an hour on foot along a route of roughly four kilometers.

Somewhere in the middle of that stroll you will have passed through the entire length of a sovereign country, with its own head of state, its own flag and its own seat at the United Nations.

Most people would clock it as an expensive neighborhood. It is, on paper, a nation.

A country the size of a park

The numbers are hard to hold onto because they keep undercutting your sense of what a country is. Monaco covers about two square kilometers. New York’s Central Park is larger than that at more than three square kilometers, so an entire independent state would fit inside a single city park with room to spare. Only Vatican City is smaller. And into that space are packed nearly 39,000 people, making Monaco a crowded patch of land, a vertical crush of apartment towers stacked up a hillside that runs almost straight into the Mediterranean.

Small countries are usually small because they are remote, or poor, or leftovers of some older border dispute. Monaco is the opposite. It is small and the world’s top-ranked city for wealth per capita, and the two facts are not a coincidence. The smallness is part of the machine.

The densest money on Earth

 In Monaco, more than 40 percent of residents are millionaires. Not 40 percent of homeowners, or 40 percent of some elite slice. More than two in every five residents. There is no other city where millionaires make up such a large share of the population.

Walk that hour-long line across the country and, statistically, roughly two in every five people you pass could hold at least a million dollars in liquid investable wealth.

The reason they are there is not the weather, or not only the weather. It is the tax. Monaco has not levied a personal income tax on most of its residents since 1869, with the notable exception of French nationals covered by a tax treaty with France.

For someone with a large income or a large fortune, becoming a Monaco resident can sharply reduce the personal income tax they would owe elsewhere, depending on their circumstances. Concentrate that incentive on two square kilometers of Riviera coastline and you get exactly what you see: the wealthy of the world clustering into a space too small to hold them, bidding up every apartment, because the address itself is the asset.

So the density is engineered. The country is small enough that living there is a deliberate, exclusive act, and the tax rule can make that act pay for itself. Scarcity of space plus absence of personal income tax for most residents equals the richest crowd on Earth standing shoulder to shoulder.

What the concentration is actually for

What I keep wondering, looking at all this, is what the concentration buys. Not for Monaco, which does very well out of it, but for the people doing the concentrating.

I am not a finance person weighing in on tax policy, just someone who has watched, in a much smaller way, how the lifestyle markers people chase tend to land. In my own life the things I aimed at for years, a certain income, a nicer car, the next upgrade, were reliably flatter on arrival than they were in the wanting. The car was a car by the second week. I suspect that pattern does not switch off at the top; it just gets more expensive to keep testing.

Monaco is that test run at the highest possible resolution. The mild weather, the low personal taxes, the exclusive address, the neighbors with the biggest boats. It is a place built almost entirely out of the things people probably imagine will finally be enough, gathered into one small, sunny, crowded strip.

Whether it delivers the feeling the money is chasing is not something a visitor can read off the harbor. But it is worth asking, because most of us are running a low-budget version of the same experiment, and the answer probably rhymes.

You can still just walk across it in an hour, though, which is the part I like best. A whole country, the wealthiest city-state per person on Earth, and you can see it end to end on foot before lunch.