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I've seen a few situations where the different ways to write numbers with more than three digits has become a small topic of discussion itself within comments.

  1. One case is the use of a period versus a comma as a demarkation between the whole number and decimals. For example pi could be written either as 3.142 or 3,142 depending on where in the world the author has studied.

  2. Similarly, An Astronomical Unit is exactly 149597870700 meters. I'd write that as 149,597,870,700 meters, but it's written in Wikipedia as enter image description here using thin spaces in lieu of comma delimiters.

What are all of the acceptable ways to do this here, and if possible what might a good set of personal options be for me to adopt to be the most user-friendly to the widest variety of international readers here?

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The SI standard says:

Following the 9th CGPM (1948, Resolution 7) and the 22nd CGPM (2003, Resolution 10), for numbers with many digits the digits may be divided into groups of three by a thin space, in order to facilitate reading. Neither dots nor commas are inserted in the spaces between groups of three. However, when there are only four digits before or after the decimal marker, it is customary not to use a space to isolate a single digit. The practice of grouping digits in this way is a matter of choice; it is not always followed in certain specialized applications such as engineering drawings, financial statements, and scripts to be read by a computer.

The advantage of this is that it's unambiguous, whereas commas and dots are used alternately as decimal separators and thousands separators in different cultures.

I've been using the HTML character entity   to produce a thin space (e.g., 10 000). HTML character entities work in Stack Exchange questions and answers, but not in comments. However, it seems that browsers treat &thinsp as a breakable space, so you might end up with 10
000 if you're unlucky. There is a non-breaking thin space, which you can get with   (e.g., 10 000), but that's kinda hard to remember... The character entity   is non-breaking, but it's a full-width space, which isn't appropriate for this usage.

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    $\begingroup$ +1 Better than either commas or periods. Those are not confusing when there's more of them (149,597,870,700 or even 149,597,870,700.25), but they are when there's only one (149,597) and we have to get from the context if this is one hundred and fortynine thousand something or one hundred and fortynine something. By using spaces as thousand separators people can keep using whatever they prefer for decimal separator. $\endgroup$ – Jan Doggen Dec 19 '18 at 16:14
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    $\begingroup$ I've used the "fancy, hard-to-remember, non-breaking" thin space &#8239 here and nothing blew up! Thank you. $\endgroup$ – uhoh Dec 31 '18 at 1:33

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