The set of rational numbers, represented by the symbol $\mathbb{Q},$ is formed by all numbers that can be expressed as the ratio between two integers, $\frac{a}{b},$ where $b\ne 0.$ $$\begin{array}{c}\text{-}3,\frac{1}{3},5,\frac{31}{45}\end{array}$$ All four of these examples are rational numbers. Note that integers are also rational, because they can always be written as fractions with a denominator of $1.$ $$\begin{array}{c}\text{-}3=\frac{\text{-}3}{1},5=\frac{5}{1}\end{array}$$ Real numbers which are not rational are called irrational numbers.