CROSS the boundaries of the former Yugoslavia and you face a few hassles. Heading from the mountains of Slovenia to the beaches of Croatia you encounter the Schengen border, which separates the European Union’s passport-free area from those EU countries which are not members of it; in holiday season this can mean an hour-long queue on the motorway. Cross from Croatia into Serbia or Bosnia-Herzegovina, and you leave the EU entirely; here you find customs checks as well as passport controls. In each new country you have to change your money.

The one thing you might not even notice is a difference in language. In Slovenia, the tourist might pick up dobrodosli, dober dan and hvala, “welcome”, “hello” and “thank you”. In Croatia, these are dobrodosli, dobar dan and hvala. Not everything is the same, by any means: prosim for “please” becomes molim. But Croatians and Slovenes can largely understand each other’s languages. This being the case, why are they considered two distinct languages at all?

Yugoslavia was cobbled together from territories mostly populated with speakers of southern Slavic tongues. It was dominated by a language called, at the time, Serbo-Croatian. Serbians wrote it with the Cyrillic alphabet, and Croats, Muslims and Montenegrins preferred the Latin one. But a few minor dialectal differences aside, they all clearly spoke the same language, its varieties closer even than Slovenian and Croatian are. Macedonians spoke another closely related language; a minority also spoke Albanian, a non-Slavic language.

But when Yugoslavia broke up and the republics went to war, not only the state but the language of Serbo-Croatian was dismantled. Nationalists in the successor republics insist today that they speak Croatian, Serbian, Bosnian and Montenegrin. Croats, in particular, started trying to purify their version of “foreign” borrowings, coining new words to replace them. Last year liberal intellectuals gathered in Sarajevo to affirm that Serbs, Croats, Montenegrins and Bosniaks speak a single “polycentric” language like English (with its standard British, American and other varieties). Nationalists howled.

Max Weinreich, a linguist, made famous the wry remark that “a language is a dialect with an army and a navy.” The usual criterion for what is a separate language, and not a mere dialect, is that speakers of two languages should find it difficult or impossible to understand each other. But factors that have nothing to do with language often supersede the linguistic ones.

Those who try to tally up the world’s languages are either “lumpers”, who would treat mutually comprehensible tongues as single languages, and “splitters”, who focus on the differences. A lumper would say that Macedonian is really Bulgarian. (Many such lumpers can be found in Bulgaria.) Macedonian splitters angrily retort that this is an attempt to obliterate their identity.

Lumper-splitter controversies can be found around the world. Swedes, Danes and Norwegians can understand each other pretty well too, but few say that they simply speak “Scandinavian”. Meanwhile Swiss-German dialects and even many dialects within Germany are more different from High German than Slovenian is from Croatian. The same could be said of Italian’s many local varieties. But most of these lack any official status.

While lines on the map are influential, other loyalties play a role too. Lumpers once recognised a language called Hindustani; its disparate speakers have no trouble conversing. But a splitter would point to the two different writing systems of Hindi and Urdu, as well as communal preferences—Muslims speak Urdu; Hindus, Hindi—and say they are two languages. Urdu is a language of India as well as Pakistan; the boundary is less territorial than religious, with the writing system a visible marker (Urdu’s Arabic script connects it to Islam, Hindi’s Devanagari links to Sanskrit and Hinduism).

In other words, the messy facts of the real world complicate the simple narratives some see as necessary for community-building. Nationalists don’t like to talk about the awful things their forebears did. They tend to assume that their country’s most expansive borders are the natural ones—though these inevitably overlap with their neighbours’ border claims. Languages are no different. A phrase beloved of some journalists is even truer of nationalists: never let the facts get in the way of a good story.