### Matrix Door

There's a striking cover image on Rajendra Bhatia's Positive Definite Matrices (Princeton series in applied mathematics. Princeton, N.J.: Princeton University Press, 2007. Mathematics Library QA188 .B488 2007 Link to MNCAT record)

This is credited on the back cover: "Doors of the Florence Baptistery show an example of a 4 x 7 matrix." Further investigation shows that these must be the gilded bronze North Doors, by Lorenzo Ghiberti (1404-24). Close-ups of the individual reliefs are available: North Doors, Florence Baptistry. The earlier south doors (by Pisano) are similarly structured, but the later east "Doors of Paradise" (also by Ghiberti) are a 2 x 5 matrix.