I recently acquired a 2,500 page, five inch thick, 1975 unabridged Webster’s Dictionary. But the massive heft and bulk belie its paucity of architectural information.

This dictionary has five entries for kern, but no definitions for the critical structural idea that spanned 1,000 years of architecture and responsible for every stone church in the Middle Ages and Renaissance.

The kern is the concept that enabled Dark Ages cathedrals to reach new heights, despite only stone masonry with no ability to resist tensile forces. James Ambrose, in my 1988 structural textbook, Building Structures, defines a kern as “a zone around the centroid of the section within which an eccentric force will not cause tension on the section.”

Simply put, a kern is the center third of a foundation. When stacking stone, its foundation will resist rotating, lifting a side off of the ground, or cracking in tension as long as the weight and forces of the structure above are within this center third.

As soon as forces become eccentric beyond that zone, tension happens. The foundation will want to twist in the ground and additional measures must be taken to avoid a failure. This is easily mitigated with steel today, but before the Industrial Age, builders had to design with the kern to ensure stability.