John Guest push fit fittings make serving from a Cornelius Keg set up easier.
Reducers for stepping down/up to different line sizes. These simply push into the fitting and then accept the alternative pipe size. Select the correct reducers stem to fit into your existing fitting and then select the correct output size.
Example, if you wanted to splice a section of 3/16 pipe into a run of 3/8 pipe, you would need 2 x 3/8 straight connectors and 2 x 3/8 down to 3/16 reducers.