uncia

Uncia meant one-twelfth in Latin. Applied to the Roman foot (pes), an uncia was one-twelfth of that foot — roughly 2.46 centimetres. Applied to the Roman pound (libra), an uncia was one-twelfth of the weight. The same word, the same fraction, two different physical quantities. Latin did not distinguish between them because the concept was the fraction, not the dimension.

The word entered Old English as ynce, then Middle English as inche. The route was direct: Latin uncia became Proto-Germanic *unkja, then Old English ynce. The modern spelling 'inch' settled in by the fourteenth century. Meanwhile, the same Latin uncia took a separate path through Old French (unce) to become 'ounce.' The two English words are etymological siblings that grew up in different households.

The length of an inch has been variously defined. Edward II of England reportedly decreed in 1324 that an inch was three barleycorns laid end to end. Whether this decree is genuine or folk history, the barleycorn definition persisted in English law for centuries. In 1959, an international agreement defined the inch as exactly 25.4 millimetres. The barleycorns were retired.

The inch is the base unit of the imperial length system: twelve inches make a foot, thirty-six inches make a yard. It persists in the United States, where screen sizes, tire pressures, and rainfall are all measured in inches. The rest of the English-speaking world has mostly switched to centimetres, but the word refuses to vanish. Every phone screen, every TV, every monitor is still advertised in inches — a Roman fraction living inside modern technology.