One side threads on to the MFL (male flare) on beer or gas keg quick disconnects (or any other male flare fitting) and the other side has a VERY SMALL 4.5mm hose barb which is best used with 4mm ID beverage tubing for a nice tight fit. The flare size is 1/4" FFL and as such is compatible with most of the flare fittings used in brewing. You can thread these on to the CM Becker gas and liquid keg QDs for example