- All integers are rational because every integer can be represented as
- Every number with a finite decimal expansion is rational (say, )
- Every number with a periodic decimal expansion (e.g. 0.314314314...) is also rational.
Moreover, any rational number satisfies exactly one of the last two conditions. The same remark holds if "decimal" is replaced with any other base.
- Rational numbers form a field. In plain English it means that you can add, subtract, multiply, and divide them (with the exception of division by ) and the result of each such operation is again a rational number.
- Rational numbers are dense in the set of reals. This means that every non- empty open interval on the real line contains at least one (actually, infinitely many) rationals. Alternatively, it means that every real number can be represented as a limit of a sequence of rational numbers.
- Despite this, the set of rational numbers is countable, i.e. the same size as the set of integers.