Stereo is similar to double buffering in that more than one image buffer is supported. Instead of front and back, left and right are provided (though generally stereo and double buffering are combined, requiring four image buffers). Special stereo video hardware alternates between scanning out the left and right buffers every screen refresh. Goggles synchronized with the vertical refresh of the screen alternately open and close LCD shutters so the left eye sees the left frame and the right eye sees the right frame. By carefully drawing the scene twice with slightly different perspective into the left and right buffers, the viewer experiences an optical illusion of 3D.
While double buffering is common on graphics workstations, stereo requires special hardware and tends to be rather expensive so many OpenGL implementations may not support stereo.