The Doric order was one of the three orders of ancient Greek and later Roman architecture; the other two canonical orders were the Ionic and the Corinthian. The Doric is most easily recognized by the simple circular capitals at the top of the columns. Originating in the western Doric region of Greece, it is the earliest and, in its essence, the simplest of the orders, though still with complex details in the entablature above.
The Doric order of the Parthenon. Triglyphs marked "a", metopes "b", guttae "c" and mutules under the soffit "d"
The Roman Doric order from the Theater of Marcellus: triglyphs centered over the end column
Original Doric polychromy
Upper parts, labelled
An order in architecture is a certain assemblage of parts subject to uniform established proportions, regulated by the office that each part has to perform.
Coming down to the present from Ancient Greek and Ancient Roman civilization, the architectural orders are the styles of classical architecture, each distinguished by its proportions and characteristic profiles and details, and most readily recognizable by the type of column employed. The three orders of architecture—the Doric, Ionic, and Corinthian—originated in Greece. To these the Romans added, in practice if not in name, the Tuscan, which they made simpler than Doric, and the Composite, which was more ornamental than the Corinthian. The architectural order of a classical building is akin to the mode or key of classical music; the grammar or rhetoric of a written composition. It is established by certain modules like the intervals of music, and it raises certain expectations in an audience attuned to its language.
Illustrations of the Classical orders (from left to right): Tuscan, Doric, Ionic, Corinthian and Composite, made in 1728, from Cyclopædia
Greek orders with full height
Doric capital of the Parthenon from Athens
Ionic capital from the Queen Elizabeth II Great Court of the British Museum (London)