Crossover Subs are the most frequently used tool in the Drill String. They allow different thread types and sizes to be connected to each other. They make up between two sizes of Drill Pipe, two sizes of Drill Collars, between Drill Pipe and Drill Collars, between Drill Collars and specialty tools, between the last Drill Collar and the Drill Bit, etc.
Depending on the two tools being connected by a Crossover, they can have a straight OD, a Bottlenecked OD, be Box x Pin, Box x Box, Pin x Pin, etc. The standard overall length for a Straight Crossover is 36”, and 48” for a Bottleneck, unless otherwise specified.