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Here, you can see the air horns in the air cap (above), and the fluid tip and needle (left), which control the fluid delivery and the fan shape. Additional holes contribute to better atomization of the fluid. The air cap can be rotated to spray a horizontal or vertical pattern. The needle size can be swapped (like changing the jets in a carburetor) according to what type of fluid is being sprayed, so you don't need different guns for different materials.