Well, the rule "For money, always decimal" isn't applied inside the Microsoft development team, because if it was: Namespace: Microsoft.VisualBasic Assembly: Microsoft.VisualBasic (in Microsoft.VisualBasic.dll) Financial.IPmt and all the other methods would receive/return decimal and not double as it is. Now I wonder if I can use this methods without worry with round mistakes? Should I use some other libraries to work with finances? If yes, could you point me some good ones (for C# use) ? Thank you.