The University of Drilling is a practical learning site, not an accredited school.

Drilling Subs

A sub and/or crossover is a drill string adapter comprising a short length of tubing with the appropriate threaded connections. The short threaded piece of pipe is used to to adapt parts of the drilling string that cannot otherwise be screwed together because of differences in thread size or design.

Display of various subs used in drill strings.
Kelly Saver Sub with a rubber protector.

 

A sub can also perform a special function, like the kelly saver sub.  A kelly saver sub is placed between the drill pipe and the kelly to prevent excessive thread wear of the kelly and drill pipe threads and can have a rubber protector which cushions adverse rotation fo the drill string.  Another sub with a special function can comprise a bent sub which is used when drilling a directional hole.