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I used to say "sharp sign" to refer to the # sign.

Today a friend told me that the correct term is number sign or hash sign or even just hash.

What is the difference between these options and what's the correct usage of the names for this sign?

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The musical sharp (♯) is technically a different character from the octothorpe (#), though they are superficially similar. The Wikipedia article describes the origin of the alternative names number sign, pound sign, and hash among others. –  choster Apr 4 at 20:15
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@FumbleFingers It’s a pound sign because of weight, not currency. Comes from grocery store practices and such. In the world of programming, in C you have pound defines and pound includes. –  tchrist Apr 4 at 20:24
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@tchrist: Maybe you do, but as a Brit I've only ever heard and used hash defines and hash includes. –  FumbleFingers Apr 4 at 20:30
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@FumbleFingers We no more use # to represent sterling than we use £ to represent weight. We consider those two separate things; perhaps you do not. –  tchrist Apr 4 at 23:15
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Related: english.stackexchange.com/q/117219 english.stackexchange.com/q/27333. Please note that octothorp was a Bell Labs innovation, and that nobody actually calls it that. Unicode calls it a NUMBER SIGN and notes that it has also been called a “pound sign, hash, crosshatch, octothorpe” in the past, and that it is distinct from a MUSIC SHARP SIGN. –  tchrist Apr 4 at 23:49

5 Answers 5

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Hashtag

Wikipedia

The use of the hash symbol in IRC inspired Chris Messina to propose a similar system to be used on Twitter to tag topics of interest on the microblogging network. He posted the first hashtag on Twitter:

“how do you feel about using # (pound) for groups. As in #barcamp [msg]?" —Chris Messina, ("factoryjoe"), August 23, 2007

enter image description here

The first use of the term "hash tag" was in a blog post by Stowe Boyd, "Hash Tags = Twitter Groupings," on 26 August 2007, according to lexicographer Ben Zimmer, chair of the American Dialect Society's New Words Committee.

Beginning July 2, 2009,Twitter began to hyperlink all hashtags in tweets to Twitter search results for the hashtagged word (and for the standard spelling of commonly misspelled words). In 2010, Twitter introduced "Trending Topics" on the Twitter front page, displaying hashtags that are rapidly becoming popular.

According to an article in The Guardian, the term octothorpe was invented by engineers at Bell Laboratories in the early 1960s. They wanted a name for one of two non-number function symbols on the first touch-tone keypads (the other was the *, which they called a sextile). The term was practically unheard of among the general public until Twitter arrived.

Oxford Dictionaries claim hash has its origin in the 1980s: probably from the verb sense of hatch, altered by folk etymology, meaning 'to cut, engrave, or draw a series of lines'.

The earliest recorded usage of the octothorpe symbol as an abbreviation or shorthand for pound, as in weight, is dated 1923 by the OED:

1923 W. E. HARNED Typewriting Stud. II. 29/1 Special Signs and Characters..#..Number or pound sign; # 10 (No. 10); 10# (ten pounds).

On a survey on keypad terminology conducted by the University of Edinburgh it was noted that the most common names for Keys to the Right of Zero were: square, hash and gate

enter image description here

Other names for the octothorpe

  • number sign (USA)
  • pound sign/symbol (USA)
  • tick-tack-toe sign/tictactoe
  • the crosshatch symbol
  • the double-cross symbol
  • hash mark/sign (UK)
  • gate (UK, and in Italian it is called cancelletto)
  • square (UK)
  • crunch
  • a symbol for fracture
  • space (in proofreading "#" indicates "insert space here")
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I'm glad you took this question on. It's good to know where/when hash was first used, and your chart is very interesting. Your answer seems to support Peter Shor's comment. Thanks, Mari-Lou. –  medica Apr 13 at 2:44
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@medica This was a rushed, botched and basically a copy'n'paste job. Your post is the best of the bunch, I was just a little amazed that nobody had really attempted to answer after a week. I made the post a community wiki one, thinking the bounty (if it was awarded to me) would go somewhere in outer-wiki-space. It hasn't. –  Mari-Lou A Apr 13 at 6:36
    
Enjoy the bounty. You deserve it. I'm glad you answered the question. ') –  medica Apr 13 at 9:19
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I'm changing the acceptable answer to yours @Mari-LouA, @Medica made an amazing answer too, but your have some amazing facts about the historic value of the sign itself. Thank you both. –  Michel Ayres Apr 13 at 15:49

In What Is the Real Name of the #?, a good explanation of this sign is given. Technically, it's called the octothorpe.

Called the pound sign, number sign and more recently the hashtag, it actually developed as a scribble for the abbreviation of pound in latin: lb, where lb is an abbreviation of libra, itself a shortened form of the full expression, libra pondo - literally “pound by weight” in Latin (though the Roman pound was only 12 ounces, not 16.)

Libra for a pound is first found in English in the late fourteenth century, almost at the same time as lb started to be used.*

If you look at how scribes scribbled lb, you might recognize the sign (in the first example) amongst the scribbled and attached lines. The reason they continued the 'b' to make it into a cross stroke was to indicate that letters were left out (i.e. it was an abbreviation.) The more careful rendition also has the cross stroke indicating abbreviation.

enter image description here

That is still how it's scribbled: I do it myself when recording the weights of babies in pounds and ounces (though we are finally moving to kilograms).

The phrase “number sign” arose in Britain because “pound sign” could easily be confused with the British currency. The # symbol is sometimes spoken as the word “number” ("number two pencil"). Another abbreviation for libra pondo became the standard symbol for the British pound in the monetary sense. Written "£", it is an ornate form of L with a cross-stroke, the way medieval scribes marked an abbreviation (from which, incidentally, we get our apostrophe). The link between pound weight and money is that in England a thousand years ago a pound in money was equivalent to the value of a pound of silver.

It appears on telephones. The name octothorp was coined by someone working for a phone company. It refers to it's eight (octo) points + thorpe (derivation questionable and possibly a joke by the person who coined it.)

Hash tag (the twitter name is twittertag comes from it's use (along with the ampersand) in IT as a tag to group information. The term twitter tag was coined by Chris Messina and popularized in a column by Stowe Boyd.

It is sometimes called the octothorn, an alternate (mis)pronunciation of octothorpe.

Sometimes it is called the tic-tac-toe sign because of the vertical and horizontal lines drawn in a game of tic-tac-toe are similar pattern to that used in #.

The sharp is slightly different in that the vertical lines are straight up-down and the cross strokes are inclined. calling # a sharp is a misnomer for the pound, or number sign.

But from the 1300s, it has been known as the pound sign, or, in England, the number sign.

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Except that a pound of silver weighs 12 troy ounces, so ~373 grams, while a pound of artichokes weighs 16 avoirdupois ounces, so ~454 grams. That’s why a pound of feathers outweighs a pound of gold in the old riddle. But I do fully agree that the # scribal abbreviation derives from libra, howsoever many ounces and of what ilk that pound should contain. –  tchrist Apr 4 at 23:32
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The pound we use to measure precious metals in has just 12 ounces not 16, just like the Roman’s version. –  tchrist Apr 4 at 23:35
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There’s also the whole set of apothecary measures, whence some of this derives. That’s why a troy ounce is exactly 480 grains and a troy pound 5,760 grains, while an avoirdupois ounce has 437½ grains and a pound avoirdupois exactly 7,000 grains. It should also be noted somewhere that despite all this octothorp silliness, the official Unicode name for the # character at U+0023 is NUMBER SIGN. –  tchrist Apr 4 at 23:43
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medica and @tchrist, there is also this question that deals with the pound issue itself (though from a different perspective). Some of the comments in particular make my non-mathemagical mind writhe and boggle. –  Janus Bahs Jacquet Apr 6 at 12:34
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I've tried looking … didn't get anywhere, although I suspect the timeline may be that the # sign replaced the lb sign on early typewriters. I would be surprised if it was a common handwritten abbreviation in the U.S. before that. –  Peter Shor Apr 11 at 23:54

The sign has multiple names and meanings:

Per Wikipedia:

The symbol is a Number Sign in North America with Pound Sign making in-roads as a name.

Outside North America it has always been called a Hash Sign.

With the advent of Twitter, hash or hashtag (named for the act of tagging with a hash sign) has become very popular in North America, too.

The Sharp Symbol in music is extremely similar, but usually looks like a hand-written version.

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Following your answer, a phrase without context would be like: "Look! A cloud shaped like a hash in the sky!" ? –  Michel Ayres Apr 4 at 20:21
    
@Michel Yes. Although you'd likely say hash sign. –  David M Apr 4 at 20:31
    
@MichelAyres At least where I live it would not be at all odd to say that (other than the fact I dont think many clouds look like # :P). That shape is generally just called a hash in much of the UK, particularly in computing and engineering communities but also in the general population. –  Vality Apr 11 at 22:36
    
@MichelAyres With a little context, it could be just hash. e.g. "hash define" (#define), "star hash one hundred hash" (*#100#), "To go to the main menu, press hash." etc. However if it really was something as abstract as a cloud shape, "hash symbol" or "hash sign" would be more likely. –  starsplusplus Apr 16 at 17:19

Octothorpe/Octothorp

Not to be confused with the Chinese character 井 (well), the sharp sign (♯), the viewdata square (⌗), or the numero sign (№)
Number sign is a name for the symbol #, which is used for a variety of purposes, including the designation of a number (for example, "#1" stands for "number one").

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I always thought that #1 was first not one –  Michel Ayres Apr 7 at 11:38
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@MichelAyres: "first" is abbreviated 1st. And #1 is pronounced "number one", never as just "one". –  Marthaª Apr 9 at 20:36
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Nobody seriously calls it an octothorpe, and nobody but a very specific kind of nerd would understand it if you said it. That name is essentially a joke that went too far. –  Bradd Szonye Apr 11 at 19:24
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A joke that has lasted 30+ years –  Third News Apr 11 at 19:38
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@Bradd is correct. When was the last time you had an automated phone system tell you "please enter your password, followed by the octothorpe"? –  Peter Shor Apr 11 at 20:42

As others have said the number sign is technically called on octothorpe. Not that anyone would know what you were talking about if you used it. Number sign, pound sign, or hash/hashtag would be more generally understood. Almost all automated phone systems will use the term 'pound' if they want you to press it on your phone.

"Sharp" is a completely different symbol to the octothorpe. Using the term "sharp" is related to music and musical notation. The terms are not interchangeable, as they look different from each other. "Sharp" looks kind of like a number sign in backwards Italics.

Compare the octothorpe (number/pound sign): #

to the sharp sign: ♯

Two different symbols.

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It's not even technically called an octothorpe, except possibly by some jokers in the telecom industry. –  Bradd Szonye Apr 11 at 19:24

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