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In English (and some other languages), we write numbers like this:

  • Four thousand, seven hundred, thirty-three and fifty-one » 4,733.51

But in Portuguese, it's written like this:

  • Four thousand, seven hundred, thirty-three and fifty-one » 4.733,51

To be clear, I'm not concerned how numbers are spelt out, but rather how their numerical versions differ in 'punctuation'.

So, where did this come from? Why is Portuguese different than English in this regard?

American English, at least

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  • I was taught in school that you shouldn't say full number names like "fifty-one" when you're talking about the numbers after the decimal point. Instead, you pronounce each digit separately. So 4,733.51 is "Four thousand, seven hundred, thirty-three point five one". The exception would be if you're talking about currency: £4.51 = "four pounds fifty-one". – GMA Aug 21 '19 at 10:36
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Historically and geographically, there's a lot of variation in decimal and thousands separators.

The decimal point is the US notation. It has become common across the world recently due to the spread of computers and calculators. But before that, a decimal point was uncommon outside the US.

The idea of a decimal mark seems to have emerged around the 12th century. Many notations were used in the following centuries in China, India, the Arabic world and Europe, including:

  • a bar over the unit digit: 3̅14
  • a vertical bar: 3|14
  • a low vertical bar: 3╷14
  • a comma: 3,14
  • a low period: 3.14
  • a raised period: 3·14

Today, most of Europe and South America as well as former French and Portuguese colonies in Africa officially uses the comma as a decimal separator.

There is also a lot of variation in thousands separators: comma 123,456, dot 123.456, apostrophe 123'456, etc. (Obviously different symbols are used for the thousands and decimal separators.) But most of the world is converging on using a (thin) space 123 456.

It is not the case that there was a worldwide standard on the period or comma and then some countries changed: there were many notations, and different countries standardized differently. Why? Accidents of history. In languages written in Latin script, I'm not aware of any language-related difference that would have favored one or the other.

Sources:

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Em Portugal embora se veja o uso do ponto para separar as sequências de três dígitos, tal é desaconselhado, devendo deixar-se um espaço: esta é, por exemplo, a habitual escrita de números em engenharia. O serviço de publicações da Comissão Europeia também o recomenda aqui. No seu exemplo será 4 733,41. Quanto ao separador decimal, a vírgula, esta secção da Wikipédia menciona as razões históricas para o seu uso. No entanto, não é raro usar-se o ponto decimal, especialmente na forma oral, penso que por influência das calculadoras.

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