A stylized version of the abbreviation for libra pondo
The abbreviation written by Isaac Newton
, showing the evolution from ℔ toward #.
It is believed that the symbol traces its origins to the symbol ℔, an abbreviation of the Roman term libra pondo, which translates as "pound weight". This abbreviation was printed with a dedicated ligature type, with a horizontal line across, 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 slash-like strokes "//". Examples of it being used to indicate pounds exist at least as far back as 1850.
The symbol is described as the "number" character in an 1853 treatise on bookkeeping. and its double meaning is described in a bookkeeping text from 1880. The instruction manual of the Blickensderfer model 5 typewriter (c. 1896) appears to refer to the symbol as the "number mark". Some early-20th-century U.S. sources refer to it as the "number sign", although this could also refer to the numero sign. A 1917 manual distinguishes between two uses of the sign: "number (written before a figure)"; and "pounds (written after a figure)".
The use of the phrase "pound sign" to refer to this symbol is found from 1932 in U.S. usage. Before this time, and still outside the United States, the term "pound sign" was used to refer to the pound currency symbol (£) or the pound weight symbol (lb). An alternative theory is that the name "pound sign" arose from the fact that character encodings used the same code for both the number sign and the British pound sign "£". Claims have included ISO 646-GB as well as the Baudot code in the late 19th century. The apparent use of the sign to mean pounds weight in 1850 appears to rule out both of these code sets as the origin, although that same reference admits that the earliest reference in print was a decade after Baudot code.
"Hash sign" is found in South African writings from the late 1960s, and from other non-North-American sources in the 1970s.
The symbol appears to be used primarily in handwritten material, while in the printing business, the numero (№) symbol and barred-lb (℔) are used for "number" and "pounds" respectively.
For mechanical devices, it appeared on the keyboard of the Remington Standard typewriter (c. 1886), but was not used on the keyboards used for typesetting. 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.