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When preparing a technical report or paper, is it better (more readable, more common) to use a space between each value and the units for it, or no space?

With space: 10 mA
Without space: 10mA

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    I'm voting to close this question as off-topic because it is not about academics or academic publishing, but about how to format scientific text for publishing. – RoboKaren Sep 24 '15 at 0:54
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    Due to preferences of my co-authors I have to prepare manuscripts using MS Word. Many journals I've submitted to explicitely state in their guidelines that such documents should not contain fancy formating. Formating of numbers and units (and the space between them) is usually done during typesetting and my preferences regarding this are not taken into consideration. – Roland Sep 24 '15 at 8:56
  • @Roland: What does this have to do with anything? All options considered in the question and even all options considered in the answers are supported even by Word (without being fancy formatting). – Wrzlprmft Sep 24 '15 at 8:59
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    @Wrzlprmft I know that Word supports a thin space. Supposedly, the type setters employed by the journals know this too (I'm actually not so sure about this). However, I believe that word documents get converted to LaTeX and apparently there can be hiccups when more then the most basic formating is used in Word. My points is that a journal has the right to make their own rules regarding the issue at hand and will enforce it. Sometimes they have the rule in their guidelines, but often they don't because they take care of it themselves at the typesetting stage. – Roland Sep 24 '15 at 9:06
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  • If there is a standard in academic typesetting¹ and typesetting in general, it is to use a narrow space, i.e.:

    With space: 10 mA; without space: 10mA; with narrow space: 10 mA

    The most suitable Unicode character for this is U+202F (narrow no-break space). This is also the default setting of the LaTeX packages units and siunitx.

  • The SI Standard says (p. 47):

    a space is always used to separate the unit from the number.

    However, it does not specify the width of the space. But at least, using no space at all is against this standard.

  • As for readability, I am not aware of any studies on this, so I can only give you my informed opinion. Consider the following examples (I use a picture to ensure that we are seeing the same thing and to be able to employ justified text):

    enter image description here

    In the first sentence, no space is used. This makes it take longer to identify the unit as it merges with the numbers, in particular with long numbers.

    In the second sentence, a full space is used. Units are now read as individual words because that’s how our brain is trained to read full spaces. As our brain is also trained to process words one by one, it will first read and process 192.34 and then continue with K. In the third sentence, where narrow spaces are used, our brain is more likely to process the 192.34 K in one chunk and we also do not have the same problems as in the first sentence.

    Finally, consider the list in the second and third sentence (1 Ω, 2 Ω and 3 Ω). With a full space it becomes more difficult for our brains to group what is written into individual items, as, e.g., the first Ω has almost the same distance from the preceding 1 than from the 2 that follows it. With a narrow space, this is easier.

    Thus, using a narrow space is a compromise to have number and unit to be seperated less than words but more than the individual digits of the number.


¹ Some journals do not seem to care to make their publications consistent in this respect.

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    The siunitx LaTeX package seems to be the defacto standard. – Ben Voigt Sep 24 '15 at 2:35
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    @BenVoigt: seems to be the defacto standard – How do you measure this? In most cases, it should be impossible to see which package was used. Anyway, I mentioned the package in my answer. – Wrzlprmft Sep 24 '15 at 6:44
  • Here one stackexchange I barely see the difference between the narrow and the full space. The difference is probably so pronounced because justified text messes with the width of spaces to adjust the width of the text to fill the line. Another issue with a normal space is that if you're unlucky a linebreak will be positioned between the number and the unit. – CodesInChaos Sep 24 '15 at 8:13
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    @CodesInChaos: Here one stackexchange I barely see the difference between the narrow and the full space. – This may depend on your browser’s settings, font-rendering details and similar. I can clearly see the difference. — Another issue with a normal space is that if you're unlucky a linebreak will be positioned between the number and the unit. – There is also a full-width protected space. So, even if all you have is Unicode, you can avoid such linebreaks in all cases (full space, narrow space, no space). – Wrzlprmft Sep 24 '15 at 8:17
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    What I use in Latex is: $123 \, \Ohm. – JoErNanO Sep 24 '15 at 16:03
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The International System of Units (SI) and the NIST recommend a space.

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    A non-breaking space. I suggest that if you can't provide a non-breaking space (i) write in something better, and failing that e.g. in some cases online (ii) omitting the space is a less bad error than allowing the unit to wrap. – Chris H Sep 25 '15 at 9:18
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Different organizations may have different standards, but since you used electrical engineering units as an example, I'll highlight the 2014 IEEE-SA Standards Style Manual:

12.4 Letter symbols

In IEEE standards, letter symbols should be used rather than abbreviations. Letter symbols include symbols for physical quantities (quantity symbols) and symbols for the units in which those quantities are measured (unit symbols). The quantity and its unit can usually be separated by a non-breaking space to avoid unfortunate pagination. (emphasis added)

This document is a treasure trove of best practices for technical writing (in my opinion, at least).

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You know the problem is an answer to this might be subjective to current practice and of course editors' preferences. Easy answer is your editor will point out how they want it.

Here's what I did, I opened google scholar and typed in voltage regulation and then scanned the paper entries and you know what there were no space 12V papers, small space 12 V papers and even 12-V papers from a cursory scan of 200 IEEE entries.

So best to ask your journal or conference publication chair.

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Unless specified to do this a certain way in the Instructions for Authors, this is not your problem. It is the page editor's responsibility to make sure the paper matches the publiser's or journal's style sheet. Chances are they have some very intelligent software that makes most of this an automated process. Just follow the editorial process and make sure that anything sent to you pre-publication looks acceptable.

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