Pounds, kilograms and the story of the stone
Step on a scale in London and it may tell you your weight in stone — 14 pounds to a stone, a unit that survives almost nowhere else. It began as a literal stone weight used to weigh wool and cheese in medieval markets.
The pound itself is older still, from the Roman libra, which is why its abbreviation is lb. The kilogram arrived with the metric system: originally the mass of a liter of water, later defined by a platinum cylinder in Paris, and today by a constant of nature.
The bridge between them is exact: one pound equals 0.45359237 kilograms, by international agreement. So a 150-pound person is 68 kilograms, and a 10-stone person is 63.5 kilograms.
If you track weight, cook from recipes, or ship parcels across the Atlantic, you will meet all three. A converter keeps them straight — and keeps the medieval stone from ruining a modern diet plan.