Both 492 and .29 are written in the decimal system.
You are correct. The word “decimal” refers simply to the fact that our common number system uses only ten different numerals, or digits. With those ten single digits (0, 1, 2 . . . 9), we can count up to 9. Beyond 9 we must use combinations of those numerals, such as 1 and 0 for ten (10), and 1 and 1 for eleven (11), etc.
Do you know, or have you ever heard of, a number system for representing quantities other than our familiar 10-digit decimal system?