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Because of different applications, we need different lengths of drill stems, so couplings sleeves are used for connecting extension rods. There are semi-bridge couplings and full bridge couplings, the only difference between them is that semi-bridge couplings have a middle stop at the middle of couplings. Usually couplings two ends are the same couplings, like R25, R32,R38,T45, T51, etc .While sometime people need crossover couplings to work, for example, one end is R32 and another end is R38, but basically all couplings are female threads at the two ends.