Why does the comma in 15,000 won come every three digits?

In English, 1,234,567,890 is read as 1 billion 234 million 567 thousand 890. Each comma lines up exactly with one reading unit (thousand, million, billion). Since English units grow by a factor of a thousand, the comma tells you where each unit starts.

So an English speaker can simply follow the commas and read aloud. Ten digits in a row with no commas are easy to miscount by one place, and the commas spare you that effort.

Read in Korean, the same number is 12 eok 3456 man 7890. Man, eok and jo each rise by a factor of 10,000, so the comma positions do not match the reading units. To read a big number you count digits from the back to find the man and eok. Even with the comma in 15,000 won, you probably convert it in your head to "1 man 5 cheon won" (10,000 + 5,000).

Article 44 of the Korean orthography rules also says numbers should be written with spaces at man-unit boundaries (12 eok 3456 man 7898). That is why some newspapers write "1만5000원" without commas. Some people say commas every four digits, as in 12,3456,7890, would read more easily, but that style is not widely used.

In its 1948 and 2003 resolutions, the General Conference on Weights and Measures said that when digits are split into groups of three, a space is used and neither commas nor dots are inserted, as in 1 000 000. The SI Brochure adds that such grouping is optional and is not always followed in fields such as financial statements. France uses a comma as the decimal marker, and Germany uses a dot between thousands, which is how forms like 1.234,56 appear.

Still, the three-digit comma has become customary on price tags and bank statements. When you meet a number mixing dots and commas on a foreign website, it is safest to check that country's notation first.