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Answers to Correct usage of lbs. as in "pounds" of weight suggest that "lb" is for "libra" (Latin), but how has this apparent inconsistency between the specific unit of weight "pound" and the more general concept of weight "libra" arisen? It seems analogous to abbreviating "grams" to "w" for weight which makes no sense to me.

Update: very nice historical context provided in your answers. Interesting that the Latin term has evolved into both pound (weight) in English and liter (volume) through Greek. I wonder if this is related to how "ounce" can refer to both weight and volume (fluid ounce)?

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‘Ounce’ just means ‘one twelfth’ (though the reason why this is so is obscure), so it has been used to refer to one-twelfths of many different things—even time (1 ounce = 7.5 seconds)! It is also the origin of the word ‘inch’ (a twelfth of a foot). –  Janus Bahs Jacquet Aug 2 '13 at 15:28
    
@JanusBahsJacquet - If 'ounce' means one twelfth, what is 7.5 seconds an ounce of? Unless I'm missing some somewhat obscure reference, shouldn't an ounce be 5 seconds? –  Shauna Aug 2 '13 at 19:37
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As to why ounce/uncia is "1/12th", that is related to the Roman system of measurement. Duodecimal systems were not infrequently used around the Mediterranean, but the exact development and origin of why the Romans divided an as into 12 uncia is not entirely certain. See this question for more information: Why is 1/12 called an “uncia” in Latin? –  Cerberus Aug 2 '13 at 19:46
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@JanusBahsJacquet If "ounce" means 1/12, then why is it used to describe 1/16th of a pound? As for fluid ounces, I'm also not sure what one fluid ounce is 1/12th of either - it's 1/8th of a cup and 1/16th of a pint. Maybe the answer is really "the Imperial measurement system makes no sense at all" :) –  SqlRyan Aug 2 '13 at 22:38
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Early English pounds were all 12 ounces, either troy weight (12 × 480 = 5,760 grains) or tower weight (12 × 450 = 5,400 grains), based on two common coinage standards. Merchants developed the “mercantile pound” (15 tower ounces = 6,750 grains), the Hanseatic pound (16 tower ounces = 15 troy ounces = 7,200 grains), and the wool pound (16 ounces avdp = 7,000 grains). It's not clear why they inflated to 15 and 16 ounces – perhaps they wanted easy divisibility by five, or they commonly weighed things closer to a multiple of 7,000 grains. –  Bradd Szonye Aug 3 '13 at 19:41
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4 Answers

Lībra in Latin originally means ‘stone’, thence ‘pound weight’ (i.e., the little stone you put on scales to weigh things), thence ‘pound’ (the weight of one of those stones), and only from that was the meaning generalised to mean ‘weight’ in general. The phrase lībra pondō ‘a pound by weight’ was then used to refer unambiguously to the second meaning.

The second word in that phrase, pondus ‘weight’, was borrowed from Latin into some stage of Common Germanic at some point, but in the narrowed sense of lībra pondō—i.e., cutting off the ‘wrong’ half of the sentence. This is then the source of the English word ‘pound’. Similarly, an earlier, Proto-Italic, form of the word lībra (*līðra) was borrowed into Greek, where it became used as a specific measure of liquids: λίτρον/λίτρα ‘litre’.

This is quite similar to how ‘kilo’ (meaning ‘thousand’) has been generalised to a kilogram (a thousand grams), except the latter is a much later development. Or to how ‘mile’ (meaning also ‘thousand’) has come to mean a specific measure of length (originally mīlle passus ‘a thousand paces’ or mīlia passuum ‘thousands of paces’).

It is very common for generic units of measurements to become fixed in meaning to a specific measure of that unit; and also for determiners of measurements to become fixed in meaning to the measurement they are determining.

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Nitpick; I believe it was actually milia passuum. But +1 anyhow. –  TimLymington Aug 2 '13 at 10:54
    
Oops, I got my singular and plural mixed up there. I’m not actually sure whether the singular (mīlle passus ‘a thousand paces’) or the plural (mīlia passuum ‘thousands of paces’) was the form standardised to mean ‘mile’; but it definitely wasn’t mīlle passuum, fixing that! (And you should +1 the answer if you say you will. :-þ) –  Janus Bahs Jacquet Aug 2 '13 at 11:02
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Same thing happened with minuta prima and minuta secunda, from which we get "minute" and "second". –  OrangeDog Aug 2 '13 at 13:10
    
Ah, never mind. I was wondering why you said pondo while the classical ablative would be pondere, but it appears pondo is an idiomatic adverb in classical Latin, from an archaic ablative pondo. –  Cerberus Aug 2 '13 at 19:34
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Related to this question is the fact that another symbol was also originally derived from the word libra.

This is the symbol £, which is an ornate letter "L" (from libra), now used to denote the Pound sterling, more commonly referred to as the British pound (ISO code GBP).

Prior to 'decimalisation' in February 1971, British currency followed the structure that had been used since pre-800 AD. (Further details in this answer.) The Pound sterling, being the main currency unit, continued unchanged from pre-decimalisation into the current decimalised currency.

The name Pound and the symbol £, originate from the fact that the value of the Anglo-Saxon pound was equivalent to one pound weight (libra) of silver.

The association of the word sterling with the British pound originates from shortly after the Norman Conquest of England in 1066, but the actual etymology is unclear. Subsequently, the composition of the silver used to make the coinage was specified by King Henry II, in 1158, as 92.5% pure silver, which subsequently became known as Sterling silver.

The pound sterling is the world's oldest currency still in use.

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The £ character is called the pound L in typesetting - or at least it used to be in my day. –  OldCurmudgeon Aug 2 '13 at 22:11
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And prior to decimalisation, of course, a common abbreviation for pounds, shillings and pence was 'l.s.d'. –  scottishwildcat Aug 3 '13 at 9:15
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This isn't really about English, but Latin. However, since Latin.SE hasn't got off the ground yet, here is as good a place as any to answer.

Librum is Latin for 'a pound in weight'. It is also Latin for 'a pound weight' (that you put on one side of the scales), and 'weight' as an abstract concept. (The three are closely connected in all languages; whether the Romans actually distinguished between them would be an interesting question for Latin.SE.) Neither of the latter two words really need an abbreviation, but the third clearly does, so lb. is generally understood as standing for 'pounds avoirdupois'.

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Not to mention the fourth meaning: a (set of) scale(s), a balance. :-) –  Janus Bahs Jacquet Aug 2 '13 at 11:10
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Libra (both singular and plural, and abbreviated lb), was a standard of weight used by ancient Romans. The Latin word may encompass the English for "pound," "scales," or "weights".

See:
- http://dictionary.reference.com/browse/libra
- http://translate.google.ca/?hl=en&tab=wT#la/en/libra

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Welcome to ELU and thanks for your contribution. May I draw your attention to the section 'Provide context for links' on the Help page. I realise that you have provided some information from the links, but your original format looked as if the links were your main answer, rather than being sources for your answer. I've therefore taken the liberty of rearranging your answer. Thanks. –  TrevorD Aug 2 '13 at 14:30
    
Thanks Trevor. Much obliged. –  Otto Cortés Aug 2 '13 at 14:56
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