The Keystone at Chartres

Marie-Claire was seven years old the first time her grandfather took her inside Chartres Cathedral. She remembers tilting her head back until her neck ached, staring up into the ribbed vaults soaring a hundred and twenty feet above the nave floor. "How does it stay up?" she whispered. Her grandfather, a retired stonemason, knelt beside her and pointed to the center of the nearest arch.…