Why do we put a comma after three digits when using big numbers?

[leroyjenkens]

It doesn't matter. Whichever came first obviously influenced the other one. They probably evolved at the same time.

Not necessarily. People could have just been writing 1 million as 1000000 and then someone could have come up with the good idea later to put commas in. That guy could have chosen to put a comma in after every 4 digits, but didn't because it didn't look right.