Abstract: An articulation has two cheeks 24, 26, intended to be fixed to a seat part and a backrest of the seat. The cheeks can rotate with respect to each other about an axis X. The articulation also includes structure 30 for coupling the cheeks together, which are actuated by two rings 42a, 42b centered substantially on the axis. The rings can be moved in two opposite directions for respectively adjusting the inclination of and for folding down the backrest, between a first position in which at least one coupling member 34a, 34b is secured to the first and to the second cheek respectively, and a second position in which the coupling member is released. The rings each comprise a stud 48a, 48b formed on their interior contour, and a spring 50 hooked onto the studs to elastically return the rings in opposite directions, into their first position.