Thanks macropod, Dana DeLouis, Niek Otten, and ~L for your quick responses.

Evidently even though Excel allows the selection of 30 digits display to be
made, it only uses 15. My mistake before when I said 12. What that means to
me is that if the result of the cell formula is one third, that it will
display as 0.333333333333333000000000000000 and only 15 digits will be used
in any further calculations. I do see that there are several packages that
can be downloaded and promise to allow greater precision calculations under
Excel. I'll research them.

This does leave one question in my mind. Why would Excel allow the display
of 30 digits when it just plugs in zeros for the last 15?