Quandaries and Queries when writing a large number, why do we insert a comma every three numbers instead of every two or four numbers? linda Hi Linda, I went to The Earliest Uses of Various Mathematical Symbols and found that the earliest example of the system of separating the numeral into groups of three with commas appeared in 1795, so we have been doing this for a long time. I found it under the "Grouping symbols" link. Not everyone uses a comma to separate the numbers, in many European countries they use a space.For example where you would write 2,453,567,890 many people would write 2 453 567 890. But your question was why groups of three rather than two or four? I have never seen a reason given for this but I can offer a possible reason. There is something special about the number three. If you show someone three objects they can tell you immediately there are three, without counting. For most people, if you show them more than three they have to count. Also research by psychologists has shown that babies, as young as three or four days old, can tell the difference between collections of two and three items. That's the best I can do. Harley Go to Math Central