Be sure you order the correct size connectors and tube for the replacement filters for your drinking water system.

These two filters with the blue and orange "sanitary plugs" are female threaded and require a threaded fitting to be screwed into it.  The sanitary plugs are to be removed and discarded as the connector is installed.  The blue sanitary plug indicates 1/4" FNPT and the orange is 3/8" FNPT (Female National Pipe Thread) standard.

The following two filters show a 1/4" quick connector and a 3/8" quick connector.  Tube size is measured by the outside diameter of the tube.  Quick connect fittings are measured on the inside of the port.