Number Sign - Wikipedia

It is believed that the symbol traces its origins to the symbol ℔,[a] an abbreviation of the Roman term libra pondo, which translates as "pound weight".[7][8] The abbreviation lb was printed as the dedicated ligature ℔, including a horizontal line across (which indicated abbreviation[9]).[8] Ultimately, the symbol was reduced for clarity as an overlay of two horizontal strokes = across two slash-like strokes //.[8]

In printing, a sign similar to # is used in as a correction symbol in margins to indicate a space is needed between two words, as noted in Joseph Moxon’s 1683 book Mechanik Exercises[10], in Philip Luckombe’s 1770 book on printing[11] or the 1847 revision of Noah Webster’s Dictionary by Chauncey A. Goodrich.[12]

A similar sign (like ⌗) has also sometimes been used as a sign for the ducat coin,[13][14] other times included with abbreviations starting with D, or as a substitute for the milréis sign.[15] In some 16th century German accounting manuscripts, the numero sign (№) is written with an N with two extra lines ornamenting it.[16]. The symbol, printed as ♯, is described as the "number" character in an 1853 treatise on bookkeeping,[17] and its double meaning (number, pound) is described in a bookkeeping text from 1880.[18] German language references from 1873 or 1892 also shows the symbol for 'ducat' (Ducaten) or 'number' (Nummer), printed as ♯[19] or ⌗, with number also represented with ⌗º.[20][b]

The instruction manual of the Blickensderfer model 5 typewriter (c. 1896) appears to refer to the symbol as the "number mark".[21] Some early-20th-century U.S. sources refer to it as the "number sign".[22] A shorthand textbook written in 1903 refers to this symbol as the "pound or number sign" and details its two distinct uses (before and after a number).[23] A 1917 manual distinguishes between two uses of the sign: "number (written before a figure)" and "pounds (written after a figure)".[24] The use of the phrase "pound sign" to refer to this symbol is found from 1932 in U.S. usage.[25]

For mechanical devices, the symbol appeared on the keyboard of the Remington Standard typewriter (c. 1886).[26] It appeared in many of the early teleprinter codes and from there was copied to ASCII, which made it available on computers and thus caused many more uses to be found for the character. The symbol was introduced on the bottom right button of touch-tone keypads in 1968, but that button was not extensively used until the advent of large-scale voicemail (PBX systems, etc.) in the early 1980s.[4]

One of the uses in computers was to label the following text as having a different interpretation (such as a command or a comment) from the rest of the text. It was adopted for use within internet relay chat (IRC) networks circa 1988 to label groups and topics.[27] This usage inspired Chris Messina to propose a similar system to be used on Twitter to tag topics of interest on the microblogging network;[28][29] this became known as a hashtag. Although used initially and most popularly on Twitter, hashtag use has extended to other social media sites.[30]