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In the book "Scientific Style and Format: The CSE Manual for Authors, Editors, and Publishers" it says:

The octothorp ("8 fields" ) has been used in cartography as a symbol for "village "... .

But the octothorp, as a number sign, is used in the U.S. to signal an apartment or unit within a block at a particular address. For example:

Mr. M Murphey
72 President St #4

Is it possible that this use of the octothorp has its roots in its use in older cartography to mark the site of a village?

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Isn't it just that # is a symbol for "number"? Not sure how it evolved to that point, but that's basically why it's used to denote apartment units. – Adam May 31 '13 at 20:26
A theory claims that back in early 1900, the Teletype Corporation was the first to use # to mean "number". That Wikipedia article about the symbol uses the word hash 15 times, whereas it only uses octothorp 3 times (plus a few references to the alternative spellings octothorpe, octathorp, octatherp). Maybe this is a US/UK split, but I certainly wasn't aware the term octothorp was "well known". I know I wouldn't have got far on telephone IT support in the UK saying to someone "Now press the octothorp key". – FumbleFingers May 31 '13 at 22:18
The use of # for a suite or apartment number is much older than the 12-button telephone keypad. Its placement, however, is not as shown in the OP. With us, the house or building number precedes the street name but the apartment follows. The slash/virgule is not used at all. Example: 72 President St #4 – Andrew Lazarus Jun 21 '13 at 22:44
Carlo, it is used in addresses not because it is associated with addresses but is associated with numbers, and it turns out that apartments are labeled with numbers, that's all. Numbers came before apartments. See the wiki article on the number sign, which accords with my understanding. The wiktionary entry is speculative and deficient. – Mitch Jun 22 '13 at 16:24
More here: languagelog.ldc.upenn.edu/nll/?p=2461 – dotsamuelswan Feb 26 '15 at 18:34

This sign is more usually called the pound sign, hash or number sign. According to some theories, the "pound sign" stemmed from a simplification of the abbreviation of lb to mean "pound":

Historically, 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 "1"....

Ultimately, the symbol was reduced for clarity as an overlay of two horizontal strokes "=" across two forward-slash-like strokes "//".

So according to this, the hash sign which came to mean "number" (as in, "a #2 pencil" would be spoken as "a number 2 pencil") developed separately from the use of a hash sign in cartography. Similarly, in reference to apartments, the use of a hash sign is simply shorthand. Thus, "apartment number 4" could be written as "apartment #4".

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I’ve never heard it called a “hash” in the States. – tchrist Jun 22 '13 at 1:18
@tchrist: As in hashtag?! – Andrew Lazarus Jun 22 '13 at 3:47
@tchrist, Some folks haven't heard of Twitter either... – Pacerier Feb 24 '15 at 7:22
"Pound Sign" is standard Telecomm lingo "Hash" is standard Comp-Sci lingo "Number Sign" is common in general Commerce – H.R.Rambler Feb 27 '15 at 18:56
Is there also a connexion between 'hash' and 'hatch' as in 'cross-hatching'? A hash-sign does look like an extract from a cross-hatched area on a map. – David Garner Mar 1 '15 at 17:01

In a languagelog blog post, one of the commenters found in Google books a 1903 shorthand book where the pound sign # is mentioned, and it appears that even at the time, it was in the U.S. a well-established symbol for both number (#2) and weight in pounds (5#), depending on whether it appeared before or after the associated number. I expect that this usage developed in the U.S. some time in the 19th century.

It seems to have never appeared in print in the 19th century, presumably because printers used the symbols № and ℔ instead, but I would assume that it was widely used in the U.S. in handwritten documents and signs. (I am fairly sure it still was when I was young, in the 1960s and 1970s.)

The word "octothorp" started in the 1960s with a practical joke among engineers at Bell Labs that got out of hand, after which the perpetrators of the joke were apparently reluctant to tell their bosses that they had inadvertently unleashed a new word upon the world. Many, many years later, after everybody involved had retired, one of the engineers involved wrote up a detailed account of this story: The ASCII Character Octatherp, by Douglas Kerr.

The standard theory of the origin of # is that it is an alteration of ℔. This theory is certainly plausible, because it's quicker to write and looks somewhat similar. However, I don't know if there's any definitive evidence for this theory. Since the use of the # sign for weight in pounds seems to be unknown in England, this alteration probably happened in the U.S. some time in the 19th century, in handwritten documents, and this is the kind of thing that might be very difficult to catch happening in the historical record.

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Just to be pedantic, the "#" itself isn't "unknown" in the UK (it's on all our keyboards for example, and one of the non-numeric telephone digits), we just don't (generally) use it to mean "number". As others have said, we call it the "hash symbol" or just "hash". – Max Williams May 5 at 15:34
@Max: How about "pounds" (the 0.45 kg kind, not the sterling kind)? – Peter Shor May 5 at 15:36
Not sure what you're asking, but the abbreviation for pounds as in weight is "lb". – Max Williams May 5 at 15:36

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