How did the Latin of the streets turn into French, Spanish and Italian?

Slowly, over centuries, in everyday speech that hardly anyone wrote down. Classical Latin, the polished language of Cicero and official inscriptions, was never how ordinary Romans actually talked. Romans themselves had a name for everyday speech: sermo vulgaris, 'common speech'. Linguists call it Vulgar Latin. It kept changing generation after generation, while the formal written language changed more slowly and kept pulling speech back.

One lucky record shows the gap opening up. Around the 4th century AD, someone in Rome wrote out 227 common spelling mistakes with their corrections, each in the form 'right, not wrong': 'speculum, not speclum', 'calida, not calda'. The writer wanted to stamp out sloppy Latin. Instead, the list captured real sound changes, like dropped syllables, that were turning Latin into something new. Graffiti scratched onto the walls of Pompeii, frozen by the eruption of Vesuvius, shows the same everyday Latin in action, far removed from literary Latin.

By 842 AD, the gap had become unmistakable. Two grandsons of Charlemagne, Charles the Bald and Louis the German, swore an alliance at Strasbourg against their older brother. One of them, Louis the German, recited his oath not in Latin but in an early form of Old French, so his brother's soldiers could actually understand him. Copied down by the historian Nithard, that short passage is the oldest surviving document in France written deliberately in a Romance language. One scholar simply calls it the oldest French text.

Today, the languages that grew out of that drift, French, Spanish, Italian, Portuguese, Romanian and more, have over 900 million native speakers combined. They were never invented. They are Latin, still spoken in the street, changed so much over the centuries that people stopped calling it Latin.