I have enjoyed the lists of typical, and erroneous, assumptions we developers make with regards to dates and time, geography, names, addresses and genders. So I will here add a short list of erroneous assumptions commonly made about decimals and separators:

- The decimal point and the separator is globally the same.

*The wikipedia page for the decimal mark lists at least 11 different characters and combinations.*
- The decimal point is a dot and the thousand separator is a comma.

*This is the default in programming, thanks to most development principles having originated in USA. But, alas, in most European countries it is exactly the opposite.*
- At least, they are the same for every country.

*Except when they are not. Canada uses different rules depending on language.*
- What about for all situations within the same language?

*No, sorry. The Swiss use one rule currency and what is practically *the exact opposite* for other numbers.*
- I got it now. At least they are always found in the ASCII character set.

*Of course not, not even close. The Japanese prefer characters found in their own character set. Eastern Arabic numerals likewise.*
- OK, then I can assume that thousand separators are for every 3 digits, right?

*Not even close. If the Chinese or Japanese group their digits they prefer 4 digits.*
- The rules for printed and handwritten numbers be the same?

*Sorry, some countries have different traditions depending on form. Spain, Italy, Switzerland and others are the special cases here.*
- If there is a rule for separators for every X digits, they always apply. Definitely.

*Well, even the Americans seem to get this one wrong, as 4 digit numbers are often written without a thousand separator, but 5-or-more digit numbers with. This is however nothing compared to the Indians who find it natural to but a separator with before the final 3 digits, but for every 2 digits before that.*
- The separators are the same for all positions within a single number.

*Yes. Except of course if the Japanese or Chinese choose to use thousand, or rather, myriad, separators, which depend on position.*

See Wikipedia:Decimal_mark, Wikipedia:Japanese_numerals, Wikipedia:Eastern_Arabic_Numerals, How the world separates its digits and How the world separates its decimals.

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