# The Real Numbers **URL:** \ **Category:** Factual Questions\ **Created:** [November 10, 2001, 5:23pm UTC](https://boards.straightdope.com/t/the-real-numbers/92643 "2001-11-10T17:23:15Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![don\_willard](https://avatars.discourse-cdn.com/v4/letter/d/a88e57/32.png) [@don\_willard](https://boards.straightdope.com/u/don_willard)\ **Post date:** [November 10, 2001, 5:23pm UTC](https://boards.straightdope.com/t/the-real-numbers/92643/1 "2001-11-10T17:23:15Z")
In mathematics, we often see a symbol that looks like a capital R but made with double lines with space between them. What is this symbol called? Or is it just called “the symbol for the real numbers”? And what is the whole history of how they came up with it instead of using something like RR for instance, which would be easier to type than this strange symbol? signed, Resting on my Matrix Waiting for an Answer ---
**Author:** ![Reeder](https://avatars.discourse-cdn.com/v4/letter/r/c6cbf5/32.png) [@Reeder](https://boards.straightdope.com/u/Reeder)\ **Post date:** [November 10, 2001, 8:34pm UTC](https://boards.straightdope.com/t/the-real-numbers/92643/2 "2001-11-10T20:34:29Z")
You mean this? ¶ I have no idea. ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [November 10, 2001, 8:43pm UTC](https://boards.straightdope.com/t/the-real-numbers/92643/3 "2001-11-10T20:43:48Z")
Reeder, the symbol you show means “paragraph” and is commonly used by proofreaders to show that a new paragraph should start here. It’s even in the LATIN-1 character set. Sorry, don’t know the story behind the other - I think I have seen it as an uppercase R with an additional vertical line just to the left of the regular one - right? ---
**Author:** ![The\_Ryan](https://avatars.discourse-cdn.com/v4/letter/t/7feea3/32.png) [@The\_Ryan](https://boards.straightdope.com/u/The_Ryan)\ **Post date:** [November 10, 2001, 8:49pm UTC](https://boards.straightdope.com/t/the-real-numbers/92643/4 "2001-11-10T20:49:03Z")
I believe that it’s pronounced “script R”. When mathematics was being deleoped, little concern was given to how it would be typed; after all, typing has been around for far less time than mathematics. Furthermore, something like “RR” would be confusing because it looks like two symbols, when it is meant to be one symbol. If you wish to represent the real numbers in a computer document, the standard way of doing so is by use of a bold **R**. ---
**Author:** ![kgriffey79](https://avatars.discourse-cdn.com/v4/letter/k/2bfe46/32.png) [@kgriffey79](https://boards.straightdope.com/u/kgriffey79)\ **Post date:** [November 11, 2001, 1:04am UTC](https://boards.straightdope.com/t/the-real-numbers/92643/5 "2001-11-11T01:04:14Z")
I am pretty sure that is the symbol for “all real numbers”. 🙂 ---
**Author:** ![Race\_Bannon](https://avatars.discourse-cdn.com/v4/letter/r/5f8ce5/32.png) [@Race\_Bannon](https://boards.straightdope.com/u/Race_Bannon)\ **Post date:** [November 11, 2001, 2:10am UTC](https://boards.straightdope.com/t/the-real-numbers/92643/6 "2001-11-11T02:10:55Z")
I can’t answer the OP, but in the math classes I have taken, the symbol for the set of natural numbers is an N with double lines on the left side, and the symbol for the rationals is a Q with an extra vertical line inside, and on the left, of the Q. I guess there’s sort of a “double line” convention at work here. ---
**Author:** ![psychonaut](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/psychonaut/32/4655_2.png) [@psychonaut](https://boards.straightdope.com/u/psychonaut)\ **Post date:** [November 11, 2001, 2:47am UTC](https://boards.straightdope.com/t/the-real-numbers/92643/7 "2001-11-11T02:47:22Z")
The symbol is probably most often referred to as “the set of real numbers”. Originally, in mathematical typography, the symbol was simply a bold R. However, this posed a problem for mathematicians lecturing on a chalk board, as there’s no such thing as bold chalk. Thus someone started drawing letters with double lines and it caught on. Now the double-line letters are even used in typeset documents. In LaTeX, the computer typesetting system which is the most popular among mathematicians, the symbol is produced with the macro \mathbb{R}. The “bb” stands for “blackboard”. So maybe you could call the symbol “blackboard R”, though I’ve never heard it referred to that way. ---
**Author:** ![don\_willard](https://avatars.discourse-cdn.com/v4/letter/d/a88e57/32.png) [@don\_willard](https://boards.straightdope.com/u/don_willard)\ **Post date:** [November 12, 2001, 8:10pm UTC](https://boards.straightdope.com/t/the-real-numbers/92643/8 "2001-11-12T20:10:07Z")
I just like words and figured there is probably a term for this doubled up R used for the real numbers. I learned some years ago that for & there is the name “ampersand.” And we used to call \* an asterisk back in the old days, but now it is called star. # is called sharp in music and pound, I think, on telephones and in computer. Some books used to have an asterisk for the first footnote and a thing that looked like a religious cross for the second footnote andit was called a dagger. For the third footnote there was another symbol that I can’t recall. ---
**Author:** ![Peregrine](https://avatars.discourse-cdn.com/v4/letter/p/a6a055/32.png) [@Peregrine](https://boards.straightdope.com/u/Peregrine)\ **Post date:** [December 3, 2001, 1:52am UTC](https://boards.straightdope.com/t/the-real-numbers/92643/9 "2001-12-03T01:52:44Z")
I just ran across this very topic at [MathWorld](http://mathworld.wolfram.com/Doublestruck.html): “A letter of the alphabet drawn with doubled vertical strokes is called doublestruck, or sometimes blackboard bold (because doublestruck characters provide a means of indicating bold font weight when writing on a blackboard).” What you have there is a doublestruck R.