Architectural order describes the style of building. In Classical architecture, each order is identifiable by means of its proportions and profiles as well as by various aesthetic details. The style of column employed serves as a useful index of the style itself, so identifying the order of the column will then, in turn, situate the order employed in the structure as a whole.
There are three distinct orders in Ancient Greek architecture: Doric, Ionic, and Corinthian. These three were later adopted by the Romans, who modified their capitals. The Roman adoption of the Greek orders took place in the 1st century BC. The three Ancient Greek orders have since been consistently used in neo-classical European architecture.
Sometimes the Doric order is considered the earliest order, but there is no evidence to support this. Rather, the Doric and Ionic orders seem to have appeared at around the same time, the Ionic in eastern Greece and the Doric in the west and mainland. Both the Doric and the Ionic order appear to have originated in wood. The Temple of Hera in Olympia is the oldest well-preserved temple of Doric architecture. It was built just after 600 BC. The Doric order later spread across Greece and into Sicily where it was the chief order for monumental architecture for 800 years.
The Doric order originated on the mainland and western Greece. It is the simplest of the orders, characterized by short, faceted, heavy columns with plain, round capitals and no base. With a height that is only four to eight times its diameter, the columns are the most squat of all orders. The shaft of the Doric order is channeled with 20 flutes. The capital consists of a necking which is of a simple form. The echinus is convex and the abacus is square.
Above the capital is a square abacus connecting the capital to the entablature. The entablature is divided into three horizontal registers, the lower part of which is either smooth or divided by horizontal lines. The upper half is distinctive for the Doric order. The frieze of the Doric entablature is divided into triglyphs and metopes. A triglyph is a unit consisting of three vertical bands which are separated by grooves. Metopes are the plain or carved reliefs between two triglyphs.
The Greek forms of the Doric order come without an individual base. They instead are placed directly on the stylobate. Later forms, however, came with the conventional base consisting of a plinth and a torus. The Roman versions of the Doric order have smaller proportions. As a result, they appear lighter than the Greek orders.
The Ionic order came from eastern Greece, where its origins are entwined with the similar but little known Aeolic order. It is distinguished by slender, fluted pillars with a large base and two opposed volutes (also called scrolls) in the echinus of the capital. The echinus itself is decorated with an egg-and-dart motif. The Ionic shaft comes with four more flutes than the Doric counterpart (totalling 24). The Ionic base has two convex moldings called tori which are separated by a scotia.
The Ionic order is also marked by an entasis, a curved tapering in the column shaft. A column of the ionic order is nine times its lower diameter. The shaft itself is eight diameters high. The architrave of the entablature commonly consists of three stepped bands (fasciae). The frieze comes without the Doric triglyph and metope. The frieze sometimes comes with a continuous ornament such as carved figures instead.
The Corinthian order is the most ornate of the Greek orders, characterized by a slender fluted column having an ornate capital decorated with two rows of acanthus leaves and four scrolls. It is commonly regarded as the most elegant of the three orders. The shaft of the Corinthian order has 24 flutes. The column is commonly ten diameters high.
The Roman writer Vitruvius credited the invention of the Corinthian order to Callimachus, a Greek sculptor of the 5th century BC. The oldest known building built according to this order is the Choragic Monument of Lysicrates in Athens, constructed from 335 to 334 BC. The Corinthian order was raised to rank by the writings of Vitruvius in the 1st century BC.