This document is just a user manual. I do not really see an explanation there, except for a vague statement that in the case of non-arbitrary precision MATHEMATICA uses built-in double precision variables. I do not understand why, in the case of arbitrary precision, when apparently much more bits are used for storing the numbers, the extra bits are used only for improving the accuracy dynamically, but not for enlarging the range of numbers dynamically. Is there no technical description available of how the arbitrary precision numbers are implemented? Leslaw