Final answer:
The architectural feature described as small, rectangular blocks beneath the cornices of Ionic, Corinthian, and Composite orders is known as dentils.
Step-by-step explanation:
The series of closely spaced, small, rectangular blocks forming a molding or projecting beneath the coronas of Ionic, Corinthian, and Composite cornice is known as a dentil.
This architectural feature resembles teeth and is a characteristic element in classical and neoclassical architecture. It's similar to teeth in a comb or a zipper, providing a repetitive decorative feature.
Dentils are commonly used in Classical architecture as decorative elements in the entablature above the columns. They resemble small rectangular teeth or blocks that create a repetitive pattern along the cornice.