Although advertised to fit the Spa Frog bromine cartridge holder, the O-Ring fit the chlorine cartridge holder, also. The size is perfect for the application and no leaks. Over time the old O-Ring has stretched and become difficult to replace the holder once removed. With the replacement O-Ring, no more hassles when servicing the chlorine and mineral cartridges. Customer service is top of the line. Ron, I thank you for offering these replacements.