Chair having a movable seat

A chair comprising:

• a first surface comprising a first peripheral edge,
• a second surface comprising a second peripheral edge, the second surface facing with the first surface, the second surface swinging with respect to the first surface such that the second surface tilts in any direction with respect to the first surface,
• an elastic member arranged between the first surface and the second surface at least in a vicinity of the first outer peripheral edge and the second outer peripheral edge,
• a stretchable sheet material provided between the first outer peripheral edge and the second outer peripheral edge to conceal a gap made between the first surface and the second surface,
• a first groove along the first peripheral edge, the first groove opening in a first direction,
• a second groove along the second peripheral edge, the second groove opening in a second direction opposite to the first direction, and
• two deformable strips provided at two opposing ends of the stretchable sheet material such that one of the strips is pushed into the second groove and the other of the strips is pushed into the first groove to conceal the gap made between the first surface and the second surface.