Oil tubing crossover or x-over is used to connect two different types or sizes of connections.   

In the context of the oilfield industry, a tubing crossover refers to a device that enables connection or coupling between different sizes or types of tubing strings. It is used when there is a need to transition or join tubing strings of varying dimensions or threading configurations. Tubing crossovers are designed to ensure a secure connection while maintaining the integrity of the tubing system in the oilfield operations. They come in various configurations and materials to meet specific application requirements.