In masonry, it's the first stone set to join two walls...

Today, a cornerstone is a symbolic section of a building.  In olden days, it was the firsts stone set in a corner.

And sometimes had artifacts to be opened in the future.

An interesting feature when opened a hundred years later.

Large stone in the corner, often with a few words or a date chiseled into it so that we will know who put it there and when.

I believe that it was the first stone laid and used to base all measurements

a local Title company next door,

what do I know?

It is a stone used in a corner of a building during construction, often a different color or embossed with a placque. It has a hollow inside where tributes, memories of the time, small symbols, etc. are placed most often in a waterproof container for preservation. Years later the cornerstone is opened to memorialize the time when the building was built. I have placed items in a couple of cornerstones.

ceremonial building block or first stone set! You decide.

When stones are used, I believe it's the first stone set.   

Barbara has answered.

Tony and Suzanne Marriott, Associate Brokers - had missed the 'Question' part yesterday.

Now I know the answer of 'Hugelkul...'

lots of answers - guess I'm late to the party :(

It's a corner stone. 

Tony and Suzanne Marriott, Associate Brokers Perhaps, a stone connects two corners.

That's the stone in the top of an archway that applies downward pressure to keep the rest of the stones in formation.

Isn't this a tradition really now? Was the corner stone placed by a mason.

The strongest point

Just wanted to know about the "bed".

Tony and Suzanne Marriott, Associate Brokers First stone set on a foundation sometimes marked with the date for historical purposes. Could also serve as a time capsule.

That would be the stone next to the corner

I am late to the party. It looks like many others have nailed this one 

Stones used for building corners. Sometimes the first corner stone has a date carved into it depending on building.