"double bridge" within the imperial palace, would seem to be an obvious
name for the two bridges adjacent to each other. One of them is made
out of steel. The other, a stone structure, is known as a megane-bashi, or "eyeglass bridge".
In fact, the name "double bridge", is derived from the predecessor of
the steel bridge. A wooden structure with an upper and lower level,
this former bridge (1887) was refered as a "double bridge". Although
this double decker structure was destroyed, its name survided to assume
a different meaning.