A precise location is not confirmed, but they are close to the sea as Besźel is noted as having developed on a "curl of coastline" from "a port hiding a few kilometres up the river to shelter from the pirates of the shore.

"[3] Other references in the novel remain vague, but indicate the cities are to the east of Hungary, north of Turkey and are within relatively close driving distance of Varna, Bulgaria and Bucharest, Romania, suggesting a location in the Balkans near the Black Sea.

Ignoring the separation, even by accident, is called "breaching" – a terrible crime for the citizens of the two cities, even worse than murder.

The origin of this odd situation is unclear, as it started at an uncertain time in the past, perhaps before recorded European history.

Passing through the border passage takes travellers, geographically (or "grosstopically"), to the exact place they started from – only in a different city.

From a physical standpoint, little differentiates the two cities, other than slight differences of architecture, vehicles and styles of dress which citizens and visitors are trained to recognise.

If the evidence presented is convincing enough, the Committee performs whatever other investigation into the matter it deems appropriate to resolve any remaining doubts its members have.

Invoking Breach is a last resort because it is an alien power to which some consider that Besźel and Ul Qoma surrender their sovereignty at their peril.

Reviewing the book for The Guardian, Michael Moorcock concludes: As in no previous novel, the author celebrates and enhances the genre he loves and has never rejected.

Keeping his grip firmly on an idea which would quickly slip from the hands of a less skilled writer, Miéville again proves himself as intelligent as he is original.

Most impressively, Miéville’s underlying point, that all city-dwellers collude in ignoring real aspects of the cities in which they live – the homeless, political structures, the commercial world or the stuff that's 'for the tourists' – is never laboured.

[11] In April 2018, BBC Two broadcast an adaptation by Tony Grisoni, with David Morrissey as Inspector Tyador Borlú.