Why is the hash or number sign also called the pound sign?

The hash sign # is absolutely ubiquitous these days, what with Twitter hashtags being bandied around with gay abandon. But have you ever wondered why it is also called the "pound" sign?

It usually gets that name - at least in North America; it is never called that in Britain - when used in the context of a phone keypad, as in "Press the pound sign to return to the previous menu" in those interminable phone menu systems. Funnily enough, when the symbol was first added to a phone keypad, by Bell back in the 1960s, it had a perfectly good name of its own: the "octothorpe" ("octo" for the eight points of the symbol, and "thorpe" for, well, no-one's quite sure about that). But apparently that name just never took off, and it became known as the "pound sign".

Why? Well, that's also not clear. The theory is that, back in the day, the abbreviation for a pound in weight, "lb" (after libra pondo, literally "pound by weight" in Latin), was often crossed like a "t" by bookkeepers and scribes, so that it kind of looks a bit like the # symbol, and that the symbol was therefore often used instead of "lb".

Well, no, actually, a crossed "lb" doesn't look like the # symbol actually, but so goes the theory, which I have seen propounded in many places. Personally, I find this explanation singularly unconvincing, as, apparently, do others.

Another, almost equally unconvincing, theory revolves around the idea that the shift symbol on the number 3 on a British typewriter was the £ (pound) symbol. On American typewriters, this was replaced by the more useful # symbol. This may be true, but still does not explain why the different symbol also came to be called "pound".

Of course, the symbol also masquerades under a variety of other names and has a wild variety of uses:

• "hash" - this usage in computing arose in the 1970s, possibly as a bastardization of "hatch", and it has various computer usages in different programming languages. It has more recently become ubiquitous through the use of Twitter "hashtags".
• "comment" - a secondary use in computing to mean that the line following the symbol is to be read as an explanatory comment and not as an instruction within the program.
• "number" - to indicate ordinal numbers, as in "#2 pencil" or "cell block #9", a common-sense name that arose in Britain in order to avoid confusion with the British currency symbol (pound sterling).
• "sharp" - an italicized version of # used in musical notation for centuries to mean "make the note a halftone higher".
• "space" - used in copy editing to mean "insert a space between these words or sentences".
• "equal and parallel" - a symbol used in mathematics and geometry. The symbol also has other uses in mathematics, including the cardinality of sets in set theory, the connected sum of manifolds in topology; the primorial function in number theory, etc.
• it also has several other, less official names, including "square", "gate", "crunch, "tic-tac-toe", etc.