why are manhole covers not square?

For those of you that have been puzzling over this strange fact on our business cards.

think differently ...

Think what could happen when a heavy manhole cover is being moved around as it is lifted up or placed back over the hole.

If a square manhole cover was turned on its edge along the diagonal of a square manhole, the cover could fall through the hole.

Many manhole covers are circular. One explanation offered for this phenomena is that the manhole cover is then always wider than the width of the hole it sits over, avoiding possible dropping accidents.

Can you think of any other possible explanations? Go on, start studying the shape of manhole covers around you.