One order, many authorities
The same order as drawn by Vitruvius, Serlio, Vignola, Palladio and Scamozzi, side by side.
The rules it illustrates
Proportion is natural, like harmony. Hold that the beauty of the orders comes from fixed proportions that please by nature, as consonances do in music; where the ancients differ, the differences answer to circumstances such as distance and height.
Positive and arbitrary beauty. Distinguish positive beauty, which pleases everyone for evident reasons (fine materials, size, precise workmanship, symmetry), from arbitrary beauty, which pleases through custom, including the exact proportions of the orders.
Where it appears in the book
In the chapters The Authorities Compared.