Highlighted range identifier shows as 10R x 3C instead of A1:C10

[rwsrws]

Excel 2007
With reference to the Name Box to the left of the Formula Bar:
All of a sudden when I highlight a range on screen with the mouse it gives an R1C1 type of display instead of the A1 type. For example, if I highlight A1:C10, it displays as 10R x 3C as long as I have the mouse button clicked, then reverts to A1 when I let go.
I did not knowingly do anything to cause this. How do I get the normal A1:C10 back?

Thanks,
rwsrws