To find the length of each side of a square chess board given its perimeter, you can use the formula for the perimeter of a square, which is:
\[ P = 4s \]
where \( P \) is the perimeter and \( s \) is the length of each side.
Given that the perimeter \( P \) is 72 inches, you can set up the equation:
\[ 72 = 4s \]
To find \( s \), divide both sides by 4:
\[ s = \frac{72}{4} = 18 \]
So, each side of the square chess board is 18 inches long.