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A crossover sub is used to crossover from one connection size to another or as the disposable component used to extend the connection life of a more expensive drill stem member. Cross-over sub is mainly a short sub assembly which is used to enable the connection of two components with differing thread types or sizes.
Couplings are short subassemblies used to enable two components with the same connection and size to be connected. Couplings are also known as loose coupling and spare coupling.
Pup joints are non-standard lengths of piping used to adjust the height of full length tubing, or casing strings.