A chair comprises a seat and a seat back made of non-resilient materials.The seat has a shape that matches the natural contours of an occupant's buttocks, thighs, and upper legs, wherein: The seat's concave backward portion connects to the concave lowest portion of the said seat back, creating a dedicated concave zone that contours the occupant's buttocks closely； the seat's concave backward portion has in its center a small hump that contours tightly an occupant's sacrum and coccyx if his buttocks are fitting inside the dedicated concave zone； the seat's forward portion has a wide W-shape that contours the occupant's upper legs: the two concave zones of the seat's W-shaped forward portion contours the back sides of the occupant's upper legs, the center zone of the seat's W-shaped forward portion is convex and snuggles the occupant's thighs； the seat is inclined backwardly in such a way that, when the occupant intentionally sits on the forward portion of the said seat, his buttocks, pushed by the gravity force of his body, will gradually glide backward and fit themselves into the said dedicated concave zone eventually； the left side and right side of the seat's W-shaped forward portion are convex so that they contour the outer sides of the occupant's upper legs.The seat back contours the occupant's back, wherein: The seat back's lowest portion is concave to contour the occupant's buttocks； the middle portion that is convex to contour the lumbar vertebrae； the upper portion that is concave to contour the thoracic vertebrae； the left and right edges of the seat back are convex so that the occupant's back cannot bend left or right therefore must stay straight up； the seat back is height-adjustable to ensure that it well contours the occupant's spine； the seat back is positioned in a way that the occupant's back stays at an angle close to verticality, with his upper legs at an angle that is slightly greater than 90 degrees to the body's alignment.
