Since $\frac{9 \pi}{4}$ is in the fourth quadrant, we can find the reference angle in the first quadrant, which is $\frac{\pi}{4}$.
In the first quadrant, the tangent function is positive, so $\tan \frac{\pi}{4} = \boxed{1}$.
The answer is: 1
Below is a math problem. Provide a detailed, step-by-step solution.

### Problem:
Find $\tan \frac{9 \pi}{4}.$

### Answer:
