Hinge for a vehicle seat

The adjustment unit (105) fitted to a vehicle seat, to adjust the pitch of the backrest, gives a relative movement between the upper (111) and lower (109) sections of the seat to give a free central swing movement to the backrest through a swing section (114). The free swing section is at a locking unit (135) at the upper seat section, secured to the lower seat section by a slit and pin system (145,147). The locking unit is a ring around the swing axis of the free swing section.