"Mathematica has built-in infinite-precision arithmetic. It converts \
quantities to an approximate decimal representation only when you inisist on \
it by using by the function N[] or (equivalently) //N, which does not allow \
you to specify the number of digits of precision. So the quantity Pi really \
has infinite precision, but you don't see a decimal representation of it \
until you use the N[] function."

Pi

N[Pi]

N[Pi, 100]

Pi // N

Pi50  =  N[Pi, 50]

2*Pi50