I sort of misspoke slightly, I should have said that you can't represent 0.1 in binary floating point. Much like you can't represent 1/3 in our decimal number system.

A couple of clarifications Kimberly...

If the limit is 15 digits regardless of the decimal, then the spreadsheet couldn't calculate 100 trillion?

There's no difference between round(2) and Round as shown (assuming the as shown cells have been set to 2 decimals) correct? I will not be using either of the conventions--my wife will for a specific group of folks. This group does not generally use spreadsheets--they use a calculator. This effort is to avoid conflicts wherein her spreadsheet values do not align with those of an ordinary calculator.

I'm very interested in this discussion since I never realized this stuff before and I'm sure this new knowledge, once I understand it better, will help me in the future.