In order to meet different drilling applications in mining, quarrying as well as energy exploration, we usually need different lengths of drill stems, so coupling sleeves are needed to connect extension rods. There are two types of couplings: semi-bridge couplings and full bridge couplings.
The only difference between them is that the former have a middle stop at the middle of couplings. Generally, two ends of couplings are the same, like R25, R32, R38, T45, T51, etc. But sometimes, crossover couplings are needed, for example, one end is R32 and the other end is R38, but basically all couplings are female threads.