The dot on calculator and cellphone's number 5????

It is for people who have difficulty seeing clearly to help them feel their way to the right keys. Plus in the dark it will help you find the middle number.

It was initially put there so that people who used a calculator a lot would know where the 5 was and thus all other keys on an adding machine/ That way they wuldn't have to look up from their work to position their fingers properly on the keys, and could input the numbers without looking up from the paper and thus increasing their efficiency.

I always assumed that just like the raised dots on the letters f & j on our keyboards, it's so you can find it quickly without looking. I know my cpa uses her calculator kind of like that, she can strike out those numbers while looking at them (and not totally looking at the calculator...just like we type without looking at the letters, numbers & punctuation.) I think it's the same for a cellphone, with the raised dot it's easy to find the 5 and dial without really looking for the numbers.

It's for vision-impaired or blind people. Since almost every key is right next to the 5, if you know where the 5 is, you can find your way to any other key easily even if you're blind. For the same reason, there are little 'bumps' on the F and J keys on most computer keyboards. If you know how to touch-type, you'll know your fingers are in the right position by making sure your index fingers rest on those keys. I use that as a guide even though my vision is fine, just so I don't have to look at the keyboard while I type.