[Hublium]

Wikipedia says:

>Isaac Asimov once mentioned an "interesting theory" that Romans avoided using IV because it was the initial letters of IVPITER, the Latin spelling of Jupiter, and might have seemed impious. He did not say whose theory it was.

That's the explanation I've always heard.

[garethrees]

This bit of speculation goes back at least to David Eugene Smith (1925), "History of Mathematics", volume II, page 59:

"There is a possibility that the Romans avoided IV, the initials of IVPITER, just as the Hebrews avoided יה in writing 15, as the Babylonians avoided their natural form for 19, and as similar instances of reverence for or fear of deity occur in other languages."

https://archive.org/details/in.ernet.dli.2015.201939/page/n7...

[rob74]

That would at least explain why they (sometimes) used IIII instead of IV, but apparently didn't use VIIII instead of IX, XXXX instead of XL etc. etc.