The five orders compared
Tuscan, Doric, Ionic, Corinthian and Composite at a common lower diameter.
The rules it illustrates
Column heights in modules. Make the Tuscan column 14 modules high, the Doric 16, the Ionic 18, and the Corinthian and Composite 20, the module being half the lower diameter.
The five orders as a series. Rank the orders in a sequence from the plainest and strongest to the richest and most slender (Tuscan, Doric, Ionic, Corinthian, Composite), each one diameter taller than the last.
Palladio’s orders. Make the Tuscan column 7 diameters high, the Doric 8 with its base, the Ionic 9, the Corinthian 9½ and the Composite 10; give Tuscan and Doric an entablature of about a quarter of the column, the richer orders a fifth.
Scamozzi’s hierarchy. Grade the orders by slenderness (Tuscan 7½ diameters, Doric 8½, Ionic 8¾, Composite 9¾ and Corinthian 10), placing the Composite, the “Roman” order, below the Corinthian.
Each order has a character. Use the orders according to their character: the Doric for strength and gravity, the Ionic for a middle dignity, the Corinthian for delicacy and splendour.
Where it appears in the book
In the chapters What Is Architecture?, The Authorities Compared, Reading the Treatises.