The term "order" denotes the basic style of a classical temple (see Greek Temple Anatomy). Throughout the ancient period, five standard orders developed.
The two fundamental orders are Doric and Ionic. The Tuscan order is a variation of Doric, while the Corinthian and composite orders are variations of Ionic. Elements from multiple orders were sometimes combined in the same building.
The standard Doric column has no base. Its shaft is fluted, but with sharp edges (i.e. no fillets) between the flutes (see Moulding). The Doric capital flares out to a square abacus (see Capital Anatomy).6
The Doric architrave is plain, while the frieze is decorated with alternating triglyphs and metopes.6 A triglyph is so-named because it consists of three V-shaped notches: two whole notches flanked with two half-notches. A metope, which sometimes features relief sculpture, is the space between two triglyphs. The triglyphs and metopes are complemented with studded blocks.
The Ionic column has a base. (The standard classical column base begins and ends with a thick bead, known as a torus.) The shaft is fluted, with fillets between the flutes. The Ionic capital features a pair of connected volutes (spiral decorations) topped with a very thin abacus, which is edged with egg-and-dart moulding.6 Egg-and-dart is one of several "wide-and-narrow" classical mouldings; also common are leaf-and-dart (see example) and bead-and-reel (see example).
The Ionic architrave is divided into three steps. The frieze is plain, allowing the option of continuous relief sculpture. The Ionic cornice includes a distinctive moulding known as dentils (small, evenly spaced blocks).6
The Corinthian order is essentially a variation of the Ionic, differing mainly in its column capital. The Corinthian capital features miniature volutes and staggered rows of acanthus leaves.
In Roman architecture, the three Greek orders were adjusted to suit Roman tastes. The Romans also developed two orders of their own: Tuscan, which is basically the Doric order with the column fluting and triglyphs removed, and composite, which is essentially the Corinthian order with a hybrid Ionic-Corinthian capital.11
2 - "Greek Architecture", Columbia Encyclopedia. Accessed May 2009.
3 - "Architecture: Ancient Greek", Encyclopedia Britannica. Accessed May 2009.
6 - "Greek Art and Architecture: Architecture", Encarta. Accessed May 2009.
7 - "Order (architecture)", Encyclopedia Britannica. Accessed May 2009.
9 - "Dorian (people), Encyclopedia Britannica. Accessed May 2009.
10 - "Ionian (people), Encyclopedia Britannica. Accessed May 2009.
11 - "Doric Order", Columbia Encyclopedia. Accessed May 2009.