The two diagonals of a quadrilateral have lengths $12$ and $9$, and the two diagonals are perpendicular to each other. Find the area of the quadrilateral.
1. Given that the diagonals of the quadrilateral are perpendicular to each other, we can use the property that the area of such a quadrilateral is given by half the product of the lengths of the diagonals. This is because the diagonals divide the quadrilateral into four right triangles.

2. Let the lengths of the diagonals be \(d_1 = 12\) and \(d_2 = 9\).

3. The formula for the area \(A\) of a quadrilateral with perpendicular diagonals is:
   \[
   A = \frac{1}{2} \times d_1 \times d_2
   \]

4. Substituting the given lengths of the diagonals into the formula:
   \[
   A = \frac{1}{2} \times 12 \times 9
   \]

5. Simplify the expression:
   \[
   A = \frac{1}{2} \times 108 = 54
   \]

Therefore, the area of the quadrilateral is \(54\) square units.

The final answer is \(\boxed{54}\).