one smaller nut is tightened over main nut and main nut tightened against smaller one by loosening, creating friction jamming
B.
a slot is cut partly is middle of nut and then slot reduced by tightening a screw
C.
a hard fiber or nylon cotter is recessed in the nut and becomes threaded as the nut is screwed on the bolt causing a tight grip
D.
through slots are made at top and a cotter pin is passed through these and a hole in the bolt, and cotter pin spitted and bent in reverse direction at other end