A number is considered rational if it can be expressed as the quotient of two integers, where the denominator is not zero. In other words, a number \( x \) is rational if there exist integers \( a \) and \( b \) such that:
\[ x = \frac{a}{b} \]
with \( b \neq 0 \).
So the necessary condition for a number to be rational is that it can be represented in this form. Examples of rational numbers include \( \frac{1}{2}, -3, 0, \) and \( 4.75 \) (which can be written as \( \frac{19}{4} \)).