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There are many types of fittings that are available to connect PVC pipes. Pipes and fittings can be either threaded or unthreaded. Fittings that join pipes must correspond, i.e. a threaded pipe can only connect to a threaded elbow, not an unthreaded one. Check out this Wikipedia guide that explains the different types of pipe fittings and when to use them.
Sealing PVC Pipe Fittings
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