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We often say "PIN Number", this is part of everyday conversation. But why?

PIN stands for Personal Identification Number, so what we're actually saying is Personal Identification Number Number.

Is there a reason for this?

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This is a special case of the etymological fallacy. "PIN", like almost all words in every language, has its own meaning which is divorced from its etymology (in this case, its origin as an acronym).

In fact it has two related meanings, because it is still used as a stand-alone noun, closer to its origin ("I've forgotten my PIN"), as well as in its derived sense as a modifier ("PIN number").

One of the reasons for the prevalence of the derived sense is possibly the homophony of the ordinary word "pin". It's not that it's likely to be confused with the other meaning; it's that without context it may be unclear what you are talking about, so "PIN number" serves to narrow it down and give context.

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    The homophony with "pin" may not have that much influence, though, as the contexts are clearly separate (just as when somebody says "does your card have a chip", you know they're not referring to French fries, corn snacks or casino currency). – Neil Coffey Mar 3 '11 at 15:22
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    @Neil Coffey: Agreed. Homophony doesn't help explain "ATM machine" (where you use your PIN number!) or "HIV virus". – chaos Mar 3 '11 at 16:06
  • @Neil: OK, maybe I went too far with that. Edited to reduce the weight of the point. – Colin Fine Mar 3 '11 at 16:34
  • @Neil Coffey: if someone approached you saying "I lost my pin", the statement is truly ambiguous. – Lie Ryan Sep 14 '11 at 16:46
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    @z7sg: the fallacy is in the argument that because PIN is derived from the phrase "Personal Identification Number" therefore "PIN number" is redundant. In current English you can make an argument that "PIN number" is redundant for the reason you give (and I also mentioned); but to argue that it is redundant because of its origin (as the question does) is a fallacy. – Colin Fine Sep 15 '11 at 16:30
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The so-called RAS syndrome is quite common. Other examples are "LCD displays" and "ATM machines." Technically, it is a form of tautology. It happens because people are not aware of what the acronym stands for, so they just use it like any other word.

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    Yes, it is a tautology, but because "PIN" can be used on its own with the same meaning, not because the "N" originally stood for "number". See my reply re the etymological fallacy. – Colin Fine Mar 3 '11 at 15:02
  • RAS Syndrome on Wikipedia: en.wikipedia.org/wiki/RAS_syndrome – Andy Mar 3 '11 at 17:56
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    Often people are perfectly aware of what the acronym stands for, the sentence is just easier to understand that way. – Henry Mar 4 '11 at 5:40

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