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I remember reading somewhere that if a unit is abbreviated as one character, there must not be a space between the number and the unit (e.g., 5m, 26K). If the unit is abbreviated as two or more Characters, there must be a space between the number and the unit (e.g., "10 km", "USD 5").

  • Can you please help me find the source again?
  • Is this recommendation correct?
  • 3
    Nitpick: you probably meant "10 km" (if it's about kilometres) – Jonik Sep 11 '10 at 17:52
  • I suggest that would have no use, even if you could explain it in detail. Are you really suggesting we should use 5m but 12 km? How could anyone justify that? Why not just use either SI units, or your own publisher's house style? – Robbie Goodwin Apr 25 at 21:32
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If you're typesetting SI units, it seems logical to follow the conventions of the Bureau international des poids et mesures. From the SI Brochure, §5.3.3:

The numerical value always precedes the unit, and a space is always used to separate the unit from the number. (…) The only exceptions to this rule are for the unit symbols for degree, minute, and second for plane angle, °, , and , respectively, for which no space is left between the numerical value and the unit symbol.

§5.3.7 goes on to say that “When it is used, a space separates the number and the symbol %.”

In practice, it is quite common to see non-alphabetic units such as % and °C typeset without an intervening space. I've never seen a rule that distinguished between single-letter units and longer units.

Note that the rule doesn't specify how wide the space should be. Some references recommend a normal inter-word space, while others recommend a thin space. In any case, the space is nonbreakable.

These rules need not apply to currencies, especially when they are written before the number. Specifying that single-character currencies don't take a space ($42, £42, €42) while multiple-character currencies do (AUD 42, A$ 42) doesn't feel completely outlandish, maybe that's what you remember?

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  • Fully agreed (especially about omitting the space before % and °C being common, even if non-conforming). SI specifies a thin space for thousands separation (if used) and just says space between number and unit, so I guess it would always be a normal (non-breaking) space. • The Euro is customarily written after the number: 12.34 € (with a non-breaking space) – mirabilos Jan 11 '19 at 17:30
  • @mirabilos In English € is usually written before the number. I think that's true in all English-speaking countries. In most of the Eurozone the symbol comes after the number and a non-breaking space, but this site is about English. – Gilles 'SO- stop being evil' Jan 11 '19 at 18:05
  • @mirabilos SI does not specify a thin space for thousands separation. This was a mistake by the English translator which was fixed in the meanwhile. In this answer, I write: "SI wants to prescribe the presence of spaces at certain positions, but not their exact width" – mhchem Sep 10 at 16:51
  • Ah okay, so it was updated in 2019. As I don’t speak French I cannot comment further (but I do remember looking up the English text of the spec a while ago). – mirabilos Sep 10 at 16:55
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In Canadian Style: A Guide to Writing and Editing:

  • When symbols are used, the prefix symbol and unit symbols are run
    together:

    5 cm
    7 hL
    4 dag
    13 kPa
    
  • When a symbol consists entirely of letters, leave a full space between
    the quantity and the symbol:

    45 kg not 45kg
    
  • When the symbol includes a non-letter character as well as letter, leave no space:

    32°C not 32° C or 32 °C
    

However, the International System of Units, or SI, requires a space to be used to separate the unit symbol from the numerical value, and this also applies to the symbol for the degree Celsius, as 32 °C. The only exceptions to this rule in the SI are for the symbols for degree, minute and second for plane angle, as 30° 22′ 8″. Wikipedia's style guide also follows the SI standard.

  • For the sake of clarity, a hyphen may be inserted between a numeral and a symbol used adjectivally:

    35-mm film
    60-W bulb
    

However, some other style guides, including Wikipedia's, deprecate hyphenation in these cases. The SI allows a hyphen between the numeral and the unit only when the name of the unit is spelled out, as 35-millimetre film.

Source: https://en.wikipedia.org/wiki/Space_%28punctuation%29#Unit_symbols_and_numbers

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  • How does "When symbols are used, the prefix symbol and unit symbols are run together" fit with the listed examples (that contain numbers)? – Peter Mortensen Oct 20 '17 at 7:31
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    @PeterMortensen Prefix symbol and unit symbol in this context mean things like c and m to make cm. – Williham Totland Jan 5 '18 at 14:09

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