Good question dondon
TTT the question is asked for us who wonder why there is a letter for integers without zero N (bold N that one)
and a different one Z for the set that includes zero
and also in this:
http://en.wikipedia.org/wiki/Natural_numbers
you have this:
There is no universal agreement about whether to include zero in the set of natural numbers: some define the natural numbers to be the positive integers {1, 2, 3, ...},
so go on asking Donjon and (some of us) will be pleased to help