Invalid chair comprising: - a chassis equipped with a base, - a seat movable with respect to the base and including a front-half seat and a rear-half seat which are mutually articulated about an axis A, and - a mechanism which permits articulation of the seat and is able to adopt a folded configuration, placing the seat in a down position near the base, and a deployed configuration, placing the seat in an up position at a distance from the base, this mechanism comprising: • a lower frame mounted rotatably with respect to the base about an axis B, and with respect to the front-half seat about an axis C situated near a front edge thereof; • an upper frame mounted rotatably with respect to the lower frame about an axis D situated between the axes B and C, and with respect to the rear-half seat about an axis E situated near a rear edge thereof; • a coupling arm mounted rotatably with respect to the base about an axis F, and with respect to the front-half seat about an axis G at a distance from the axis C; - an elastic return member which stresses the articulation mechanism towards its deployed configuration; the axes A, B, C, D, E, F and G being parallel and defining, in a plane perpendicular to them: - the axes B, C, G, F: a deformable lower quadrilateral BCGF; - the axes A, C, D, E: a deformable upper quadrilateral ACDE; the axes A, B, C, D, E, F and G being positioned in such a way that the lower quadrilateral BCGF is convex in the folded configuration and concave via the axis G in the deployed configuration. - a chassis equipped with a base, - a seat movable with respect to the base and including a front-half seat and a rear-half seat which are mutually articulated about an axis A, and - a mechanism which permits articulation of the seat and is able to adopt a folded configuration, placing the seat in a down position near the base, and a deployed configuration, placing the seat in an up position at a distance from the base, this mechanism comprising: • a lower frame mounted rotatably with respect to the base about an axis B, and with respect to the front-half seat about an axis C situated near a front edge thereof; • an upper frame mounted rotatably with respect to the lower frame about an axis D situated between the axes B and C, and with respect to the rear-half seat about an axis E situated near a rear edge thereof; • a coupling arm mounted rotatably with respect to the base about an axis F, and with respect to the front-half seat about an axis G at a distance from the axis C; • a lower frame mounted rotatably with respect to the base about an axis B, and with respect to the front-half seat about an axis C situated near a front edge thereof; • an upper frame mounted rotatably with respect to the lower frame about an axis D situated between the axes B and C, and with respect to the rear-half seat about an axis E situated near a rear edge thereof; • a coupling arm mounted rotatably with respect to the base about an axis F, and with respect to the front-half seat about an axis G at a distance from the axis C; - an elastic return member which stresses the articulation mechanism towards its deployed configuration; - the axes B, C, G, F: a deformable lower quadrilateral BCGF; - the axes A, C, D, E: a deformable upper quadrilateral ACDE;