A chair comprising: a seat and a seat back, the seat having a concave backward portion having a center with a hump contoured to tightly conform to an occupant's sacrum and coccyx, and a forward portion with a W-shape formed by two concave zones (202a, 202b) separated by a convex center portion; the seat back having a left edge, a right edge, a concave upper portion, a convex middle portion and a concave lowest portion connected to the concave backward portion of the seat, the concave lowest portion being contoured to conform to the occupant's buttocks, the convex middle portion has a convex middle contoured to conform to the occupant's lumbar vertebrae, the concave upper portion is contoured to conform to the occupant's thoracic vertebrae, and the left edge and right edge of the seat back are convex so that the occupant's back cannot bend left or right; the seat has its concave backward portion connecting to the concave lowest portion of the said seat back, creating a dedicated concave zone, the dedicated concave zone contours the occupant's buttocks closely; the seat is designed to be inclined backwardly in such a way that, when the occupant intentionally sits on the forward portion of the seat, his buttocks, pushed by the gravity force of his body, will gradually glide backward and fit themselves into the dedicated concave zone eventually, characterized in that the hump in the seat backward portion is in the form of a narrow protrusion extending from a front portion of the seat backward portion to a substantially vertical portion of the seat back portion, said narrow protrusion having a substantially constant cross section. a seat and a seat back, the seat having a concave backward portion having a center with a hump contoured to tightly conform to an occupant's sacrum and coccyx, and a forward portion with a W-shape formed by two concave zones (202a, 202b) separated by a convex center portion; the seat back having a left edge, a right edge, a concave upper portion, a convex middle portion and a concave lowest portion connected to the concave backward portion of the seat, the concave lowest portion being contoured to conform to the occupant's buttocks, the convex middle portion has a convex middle contoured to conform to the occupant's lumbar vertebrae, the concave upper portion is contoured to conform to the occupant's thoracic vertebrae, and the left edge and right edge of the seat back are convex so that the occupant's back cannot bend left or right; the seat has its concave backward portion connecting to the concave lowest portion of the said seat back, creating a dedicated concave zone, the dedicated concave zone contours the occupant's buttocks closely; the seat is designed to be inclined backwardly in such a way that, when the occupant intentionally sits on the forward portion of the seat, his buttocks, pushed by the gravity force of his body, will gradually glide backward and fit themselves into the dedicated concave zone eventually, characterized in that the hump in the seat backward portion is in the form of a narrow protrusion extending from a front portion of the seat backward portion to a substantially vertical portion of the seat back portion, said narrow protrusion having a substantially constant cross section.