A flexible coupling or Fernco fitting is used when joining dissimilar piping materials. This is best method to transition PVC to ABS. It is also commonly used when connecting PVC to Cast Iron drains.

A flex coupling is tightened with a special torque wrench that applies the correct amount of pressure. Flexible couplings are also commonly used when installing a sump pump. The biggest advantage of these fittings are the ability to take apart and reuse.

Flexible couplings that are used underground are required to have a shield that will protect the pipe in the event of future settling. A flex coupling is also convenient where making an alteration to an existing drain pipe. PVC and ABS are so rigid that it can be difficult to add in a fitting without having any play.