To simplify the fraction \( \frac{12}{18} \), we can find the greatest common divisor (GCD) of the numerator and the denominator.
The GCD of 12 and 18 is 6.
Now, we can divide both the numerator and the denominator by their GCD:
\[ \frac{12 \div 6}{18 \div 6} = \frac{2}{3} \]
Thus, the simplified form of \( \frac{12}{18} \) is \( \frac{2}{3} \).