The Old Sacristy was built in the first decades of the 15th century.
The sacristy is a square-shaped room covered by an umbrella dome. And considering how the height is equal to the width, you are standing in a perfect cube covered by the equally harmonious geometry of the sphere-shaped dome. As you can see, a square compartment opens up on one of the sides; if you look closely you'll realize that it's an exact reproduction of the sacristy itself on a smaller scale: it exactly corresponds to a ninth of the rest of the structure, and is also a cube covered by a dome.
You'll be struck by the simplicity and grandeur of its whole, which is the result of Brunelleschi's collaboration with Donatello, who saw to the interior decorations.
