Folding chairs of the scissor type which are made with three tubular elements 3, 4, 8 have one of said elements 3 forming front legs and carrying a backrest, another 8 forming backlegs and the third element 4 forming a seat frame. The elements are joined together by a joint device including a pair of identical rectangular pieces 1, 2, one piece 1 of which is part of the front leg and backrest element 3, the other piece 2 being presented towards the first piece 1 and being fixed to the element 4 forming the seat frame, the pieces 1 and 2 having mortising 5 forming a housing for the end of the element 8 forming the back legs, the mortising 5 having part circular shape and being open to define two flat surfaces and the end of the element 8 forming the back legs being provided with a portion complementarily shaped to the part circular shape of the mortising 5.