Collapsible chair seat with a low profile in the collapsed position

The seat has a casing (2) with a backrest (3) having proximal and distal edges (8, 9). A rotation axle (6) of a foldable base (7) is mounted on the base. The backrest has two units (3a, 3b) presenting scapular and lumbar supports, respectively. The unit (3b) slidably mounted on the casing assists in open and closed movements of the base to house the edge (9) under the scapular support in the closed position of the base.