Manhole covers - triangular halves

Manhole covers are usually made up of two pieces of triangular plates to form a square cover. One can wonder why two rectangular halves are used for a rectangular cover. To understand this, one must note that a triangular cover could basically lie on a plane whereas a rectangular cover comprises a point of redundancy. Henceforth the potential problem of rocking produced by vehicular traffic by rectangular traffic can be eliminated by using two triangular halves.  

Conversely, the two pieces of triangular covers must be bolted together. As for a piece of triangular cover, it's effortlessly dropped into the rectangular hole of manhole at the time of routine maintenance. Consequently from maintenance point of view, some nations prefer another geometrical shape which means circular, as this is the only shape that cover could barely be accidentally dropped into the manhole. Conversely for other geometrical shapes for example rectangle or square, they could still be dropped into their formed hole when inclined into proper angles.

Fig: Different types of manhole covers