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xkcd #3102: Reading a Big Number
(xkcd.com)
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Depending on handwriting or font, commas may be more visible than spaces.
Hard disagree. I work a lot with numbers, both hand-written and typed. I've yet to come across a situation where spaces are not sufficiently clear for readability. Using spaces for separation has never been an issue with letters, why would it be an issue with numbers?
the problem with spaces is that it leaves it ambigous whether it's one or multiple separate numbers, whereas a non-space separator (i think apostrophe is just obviously the best) makes it clear: 123'000'398