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The convention for units is to leave a space after the number, for example, 'the average temperature of the human body is 37.0 °C.'

But what if I need to explicitly state whether a number is positive or negative - do I use a space after the sign?

Would I write:

Set your refrigerator to + 4 °C.

or

Set your refrigerator to +4 °C.

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  • 3
    It does look a lot cleaner without the space. – Ricky Nov 20 '15 at 10:55
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    This is not an "English grammar" issue, just a matter of typography. Generally it works best to have no space between the sign and the first numerical digit. – Hot Licks Nov 20 '15 at 12:45
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    Also, I'd say it's unusual and even awkward to explicitly include the + sign. Numbers are positive by default. I'd only include the + sign in the case that there's a meaningful risk of ambiguity (eg on devices that are most frequently set to negative temps), or a risk that some prankster will pencil in a - sign unless you fill in the space. – Dan Bron Nov 20 '15 at 13:10
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    @DanBron, I agree with you - perhaps it was a poor example. My actual use case involves positive and negative use around zero, i.e., compass directions where East is -90 (or - 90) and West is +90 (or + 90). There is some real ambiguity here and statements have to explicitly include number signs as different reference frames exist. – Trojanian Nov 20 '15 at 16:05
  • Also: if you use a space, make sure it is a non-breaking space. You do not want "4" at the end of one line, and "°C" at the beginning of the next. – GEdgar Mar 14 '16 at 14:19
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Every technical writing style guide that I have seen that addresses the issue says to leave no space between numerals and mathematical symbols used as modifiers or adjectives. A few examples include:

Use a space “Before but not after + - ± ... when used as adjectives.” (Mathematics into Type: Updated Edition, 1999, American Mathematical Society, p. 39, §3.1.3c.)

“When mathematical symbols are used as adjectives, that is, with one number that is not part of a mathematical operation, do not leave a space between the symbol and the number.” (The ACS Style Guide, 3d ed, 2006, American Chemical Society, p. 215) [The listed examples include ±, >, <, and %.]

“Leave no space after mathematical operators used as adjectives” (The ACS Style Guide, 3d ed, 2006, American Chemical Society, p. 219)

“Generally, close up a number and a non alphabetic [math] symbol, whether the symbol precedes or follows the number (except if the symbol is being used as a mathematical operator).” (Scientific Style and Format, 7th ed, 2006, Cambridge University Press, p. 146)

“When these [mathematical] symbols are modifying a number rather than serving as operators, close them up to the numeral that follows or write them out.” (Scientific Style and Format, 7th ed, 2006, Cambridge University Press, p. 159)

“The signs +, -, ±, ×, and ÷, etc., are closed against accompanying figures and symbols.” (U.S. Government Printing Office Style Manual, 2008, p. 259, Rule 10.2)

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  • +1 Good answer. Do you have online references to any of these documents? – Lawrence Mar 14 '16 at 15:00
  • Thanks for the references. I intuitively use the close form because the sign seems to 'belong to the number'. For the same reason, I use the close form with ratio expressions such as '%' or '°'. they also seem to 'belong to the number'. – Trojanian Mar 14 '16 at 17:47
  • Also, I just added the publisher to the CUP publications in an edit to your answer to highlight the use of both British and American English references. Glad you used both for a more objective answer. – Trojanian Mar 14 '16 at 17:50
  • Mathematics into Type can be found at: ftp.ams.org/ams/author-info/documentation/howto/mit-2.pdf The GPO Style Manual can be viewed at: gpo.gov/fdsys/pkg/GPO-STYLEMANUAL-2008/pdf/… The ACS Style Guide can be found online, but I suspect the copies I viewed are not legal and I will not list them. – Ken Mar 14 '16 at 19:19

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