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Which key on the telephone keypad is often referred to as the “pound key”?
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If your answer to 1) is “I don’t know,” looking at the keypad which key would you guess it was?
#. Never heard anything else.
(grr. message too short)
What gap? I don’t think I’ve met anyone who didn’t know that # was the pound key.
I get a lot of Spanish-speaking customers who, I can tell from the phone display, are hitting the star key instead of the pound key. That I attribute mostly to a language barrier. But more and more lately I’ve been getting native English speakers who are making the same mistake. When I use the term “pound key” they have no idea what I mean. They do tend to understand “tic-tac-toe board” however.
What else would you call the # key besides the pound key?
The * key I usually hear refered to as the star key but I guess you could call it the asterisk key.
Yeah, it’s still pretty much the standard name.
i’ve never even heard the term “pound key”. If asked to name the # key I would say it was the hash key.
The preferred nomenclature is octothorpe 
Why “octothorpe?” There’s no “octo” anywhere near it.
…in American English. As One And Only Wanderers says, the standard name in British English is something else. Not least because ‘pound key’ for us sounds like you mean shift+3 on a UK-layout keyboard, producing ‘£’
In North America, most people seem to have no trouble identifying the pound key on a phone. It’s used all the time when navigating through automated phone systems like voicemail menus or teleconference logins.
I use it so often that the little design is starting to wear off the key. 
I always figured it was because there are eight ends coming out from the central rectangle.
That’s just as made-up as anything else.
I’m used to pound key, but I’ve also heard it called the number key.
It’s the tic-tac-toe key.
Well, you can say that about literally any word.
Pound key actually seems to be used more and more often, contrary to what I think the OP is suggesting, but some people call it the number key, since it’s often used to designate a number: Number 4, No. 4, #4.
Correct. It has eight points.
In medical settings, “#” is used for pounds (in the US, at least). As in, “Weight down 5#.”