I was writing a grocery list at the kitchen counter — 2 lb chicken thighs, 1 lb tomatoes for sauce — when it struck me that I’ve been writing lb my whole life without once wondering why. The word pound has no b in it. Where did those letters come from?
The answer reaches back two thousand years to Rome, and along the way it explains the British pound sterling and its currency symbol too. Not bad for a grocery list.
Two halves of one Roman phrase
The Romans weighed goods with a unit called the libra — their word tied to scales and balance, the same root behind the zodiac sign Libra, pictured as a set of scales. The full phrase was libra pondo, roughly a pound by weight, where pondo relates to the Latin for weighing.
English, charmingly, kept both halves and split the jobs. The spoken word pound descends from pondo; the written abbreviation lb descends from libra. So when I scribble 5 lb on my list, I’m writing one Latin word and saying a different one for the same thing.
The plural follows Latin habits too. You’ll see lbs everywhere in casual writing, though technical style prefers lb for singular and plural alike, treating it like a proper unit symbol. And the libra lives on in money: Britain’s pound sterling began as a pound weight of silver, and the £ sign is an ornate letter L for libra with a crossbar. Italy’s old lira carried the same root.
The pound is secretly metric now
The pound wandered in size for centuries — medieval Europe had tower pounds, merchant pounds, and troy pounds, all different. The winner for everyday life was the avoirdupois pound of 16 ounces, its name from Old French for goods of weight.
Its modern definition is precise and, I find, quietly funny: it’s metric. Since the 1959 international agreement:
- 1 avoirdupois pound = exactly 0.45359237 kilograms
- That’s 453.59237 grams
- Which makes 1 kilogram about 2.2046 pounds
There’s no physical pound anywhere to consult. The pound is legally a fixed slice of the kilogram — which is why the multiply-kilos-by-2.2 rule from the gym works so well.
The troy holdout
One relative refuses to conform. The troy pound, historically used for precious metals, holds 12 troy ounces instead of 16 avoirdupois ounces and weighs about 373 grams — lighter than the common pound. Gold and silver are still priced per troy ounce today, so the distinction carries real money and is worth remembering whenever precious metals enter the conversation.
So lb isn’t a misspelling. It’s a fossil — a two-letter monument to a Roman balance scale, hanging on my grocery list and on the front of the British currency symbol alike.
Next time you jot down lb, enjoy the fact that you’re abbreviating a word English never actually adopted.
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