These low-profile clips will secure any size of Blade LED to the AquaIllumination HMS Rail system. The included screws slide right into the aluminium tracks and will tighten down to hold them in place. The most common application would be 1-3 Blade Lights on an HMS Single Arm mounted on the back of the tank with the Blade resting perpendicular to the ARM. Two arms would be required for Blades 21" or longer.