A natural number is an element of the set
\NN={0,1,2,...}.

An integer is either a natural number, or its negative.
The set of integers is \ZZ.

One can complete \ZZ to the set \QQ of rational numbers
by introducing fractions of integers.

Then one completes \QQ to the set \RR of real numbers
by introducing finite limits.

And \RR is completed to \CC, the set of complex numbers,
by adding roots of polynomials.

Conclusion:

 \NN \subset \ZZ \subset \QQ \subset \RR \subset \CC

