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 $b\neq 0.$ $\begin{gathered} \text{-} 3,\quad\frac{1}{3},\quad 5,\quad\frac{31}{45} \end{gathered}$ 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 $1.$ $\begin{gathered} \text{-}3 = \frac{\text{-}3}{1},\quad 5=\frac{5}{1} \end{gathered}$ Real numbers which are not rational are called irrational numbers.