> I think the first thing that struck me as odd were the round joints and
> cylindrical tenons.  I was wondering how it wasn't tilting like a parallelogram.

I don't know if it's just the camera angles, but they look like 
elliptical tenons and joints, not circular. This might be the mechanism 
that provides the strength against folding over like a parallelogram. 
But saying that it would be no stronger than a cylindrical tenon glued 
in, so I don't know. Presumably the bases of the vertical columns are 
well and truly fixed into the ground, and not just on a pivot.

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