Cornerstone 

A Cornerstone is a large stone that is laid near the base of a new masonry building, usually in the Northeast Corner.

In modern times the Cornerstone is often placed by so-called important people.  These important people at least tap the stone with a hammer or something to indicate that they have set the Cornerstone even if it was really put there by someone else or even by a large machine.

At one time a living human was buried under the Cornerstone as good luck for the building.