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Why the disparity? And why use 'No.'? Is it from the French?

And the hash or pound sign seems a weird choice too, is there a history or any reason involved?

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No. is also used in the US. –  Matthew Frederick May 27 '11 at 10:23
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It seems to be something of an old-fashioned usage in the states though? More something you'd seen on a 'vintage' styled product. I'm think of such things as Amazon order numbers which are always prefixed by the pound sign ('#'). While we're on the topic, I'd just like to remind American users of this site that '£' is the pound sign ;-) –  5arx May 27 '11 at 10:26
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"Old fashioned" is a bit strong, but I would agree that its use is waning. Addresses tend to use # for an apartment number, for example, but it's not uncommon to see "take the No. 9 train" or "look at paragraph No. 7". –  Matthew Frederick May 27 '11 at 11:26
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So, in the UK, when they (used to) give weights in the pounds, they used £ ?? Learned something new today, I thought that was only for money. –  GEdgar Sep 6 '11 at 15:00
    
@GEdgar - not exactly. The pound sign evolved from the usage of the upper-case 'L' which was used to abbreviate the Latin word 'libra' (literal meaning 'scales') which was what mass was expressed in in Roman times. 1 libra approx. = 12oz. –  5arx Sep 13 '11 at 11:56

2 Answers 2

up vote 18 down vote accepted

No. comes from the abbreviation of "numero", ablative case of the Latin "numerus"

Also, # was used in America for an interesting reason:

In the United States, the symbol is traditionally called the pound sign or the number sign. The pound name derives from a series of abbreviations for pound, the unit of weight. At first "lb." was used; however, printers later designed a font containing a special symbol of an "lb" with a line through the verticals so that the lowercase letter "l" would not be mistaken for the numeral/digit "1". Unicode character U+2114 (℔) is called the "L B bar symbol", and it is a cursive development of this symbol. Ultimately, the symbol was reduced for clarity as an overlay of two horizontal strokes "=" across two forward-slash-like strokes "//"1

The # in America ultimately became used for numbering everything, not just in weight, but in any quantity.

The reason the # wasn't used as a numbering sign in England was because the pound in England was denoted by the (£) sign, so # wasn't used to avoid confusion.

1 The above was taken from here

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1  
I just noticed, can you provide reference for "No." coming from "number of"? –  Alenanno May 27 '11 at 22:08
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The # symbol is still used for "pound" in specialized applications, such as "24# bond paper." –  phenry May 27 '11 at 22:26
    
Idiot: did you see the comment I've sent 2 days ago? –  Alenanno May 30 '11 at 15:23
    
@alennano, edited my question. –  Thursagen May 31 '11 at 6:12
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You changed your answer, I see. :P –  Alenanno May 31 '11 at 15:50

I consider this etymology to be more correct for No., since No. is not an acronym but an abbreviation:

No.
ORIGIN: from Latin numero, ablative of numerusnumber.’ ]

I couldn't find anything on the # symbol yet, I guess that wikipedia article is the only source. I'll make sure to edit in case I find something else.

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