Why is # called the "pound" sign?

[m3]

Also, why do we have it on our phones?

[tomndebb]

The pound (lb., unit of weight) was one of several meanings given to that character. (A 3# bag of sugar, a 5# sack of potatoes.) When the phone companies were looking to add special function characters to the early push-button phones, they rejected letters, the !, $ @ ,&, etc. and settled on * and #. The phone companies and the computer companies each attempted to give it special names (such as octothorpe--a figure with eight projections), however surly programmers simply began referring to it as the pound sign and that name has stuck.

Willard Espy has written a poem giving the various names that the character is given when it is used in a wide variety of applications:

Many offices encumber
My diurnal rounds;
1. Before a digit, I'm a #; number
2. After digits, #; pound
3. In a printer's proof, a #; space
While, if at the harp
You should pluck me from my place,
4. I would be a #. sharp
5. In one game, I'm #; tic-tac-toe
6. An # on phones; octothorpe
7. In business, I'm #, although non-add
8. A # when in bones. fracture

[Harmonious Discord]

When you have to keep using it on an automated system, you want to pound the handset on the table.

[JoeyBlades]

Also, on early keyboards that didn't have the little curly 'L' (£) used to denote pounds sterling, the '#' was used instead. I see many Brits still doing this today, even though they have a '£' at their disposal.

Still, I hear many modern programming geeks use the terms:

# = hash
* = splat
! = bang
^ = hat

[Turpentine]

When I called my brother in England, an female operator's voice told me in a pleasant British accent to enter the extension and then press the "Square Sign."

[Crusoe]

Er...this may be a bit obvious, but I've used PC keyboards in several countries, and in the UK SHIFT-3 gives you £, and in the US it gives #. I'm guessing that this is also the case in older typewriter keyboards.

As for the # sign, in the UK it's generally known as the "hash" key, and is used to represent 'number' (e.g. the band "#1 Hurricane").

I wonder if this will come out correctly from my keyboard?

[Ptahlis]

Its actual name is the octothorp. Bet you don't hear that on an automated phone system!

[Wonko The Sane]

On the original phones, the spring was stronger on that button. Therefore, you had to press it harder. The reason for the harder spring was that it was the last button on the keypad. Therefore, you had to "pound the button", and the phrase was eventually changed to just the "pound button"

[Fonz]

Don't listen to these wankers.
The # and * were added to the phone pad for automated bank accounts for businesses.

The keys were used on the Teletype(r)

The # represents the "enter" on the number pad of a calculator. This is still the most common use.
The operator says, "Please enter your account number, followed by the pound sign." Sound familiar now?

The * on the teletype was an "alert" key, when there was still always a person at the other end, to wake him up that it was his turn to respond.

That was the same bell in the term "with all the bells and whistles".
The whistle on the Teletype squawked when it ran out of paper, etc.