If you happen to be a resident of Xiangyang on a particularly frosty evening, the importance of the Great Wall of China may just be dwarfed by an altogether different boundary running through the country. For unlike your neighbours just the other side of the Qinling-Huaihe Line, first drawn back in 1908, your home will not have benefited from the government-funded heating systems installed in the 1950s and allotted according to whether the property lay in the cooler north or warmer south.

The flipside — as in so many of the case studies in Invisible Lines, Maxim Samson’s study of impactful but imperceptible borders — is that almost all of the most polluted cities in China now lie above the 33rd parallel, reliant as they are on coal-fired boilers to power their heating. As a result, the average life expectancy in the north is five years below that of the south. The Qinling-Huaihe Line may be an entirely unmarked border, but it is one with real consequences.

The boundaries we cannot see, argues Samson, a professor at DePaul University in Chicago, are not necessarily any less important than the ones we can. They are “the types of lines that rarely appear on our physical or political maps and, when they do, have impacts that go far beyond what is generally shown”.