Method and system for directional drilling and coupling sub

The present invention provides a system and method for directional drilling of a borehole in a formation along a predetermined trajectory. The method comprising the steps of: providing a directional drilling system, comprising a rotatable drill string comprising a first drill string section and a second drill string section; a drill bit connected to a downhole end of the drill string; and a coupling sub comprising a first end connected to the first drill string section; a second end connected to the second drill string section; a hinge member between the first end and the second end allowing hinging of the second end relative to the first end; and locking means, operable between a hinge mode and a locking mode for locking the hinge member to prevent hinging of the second end relative to the first end; operating the directional drilling system with the locking means in the locking mode to drill a substantially straight section of the trajectory; and operating the directional drilling system with the locking means in the hinge mode to drill a curved section of the trajectory.