Superimposed orders
Arcaded storeys with Tuscan, Ionic and Corinthian columns one above another, as at the Colosseum.
The rules it illustrates
Stronger below, lighter above. When orders are placed one above another, put the stronger below and the more delicate above (Doric under Ionic under Corinthian), and make each upper column a quarter shorter than the one beneath.
Where it appears in the book
In the chapters How to Use This Book, Columns in Company.