Boltmasters Imperial BSW Left Hand Nuts feature a left hand thread, otherwise known as a reversed thread.  These threads run in an anti-clockwise direction as opposed to a standard right hand thread nut that runs in a clockwise direction.  And the BSW Hex Lock Nuts can also go by the name of Half Nut or Jam Nut.  These nuts are used to lock against a standard hex nut.  Which adds in the bolted joint becoming undone.  The height of them is normally half the size of a standard hex nut.