What are the ratio of diameters of the 5 orders of columns and what is their significance

The height-to-diameter ratio for the Tuscan column is seven diameters; the Doric is eight; the Ionic is nine; and both the Corinthian and Composite orders are ten diameters high 1.

Although all five orders are described in the lectures, special significance is attached to the three original Greek orders—the Doric, Ionic, and Corinthian—because they alone display original invention and distinct character 21. The Tuscan is treated as the Doric in its primitive state, and the Composite as a hybrid borrowing from the Ionic and Corinthian 21. Their study is introduced to encourage the craftsman in his research into architecture and proportion, and their number corresponds in the lecture to the five external human senses 21.