Ovolo is an Italian word that means "little egg". The ovolo or echinus is a convex decorative molding profile used in architectural ornamentation. Its profile is a quarter to a half of a more or less flattened circle.
The 1911 edition of Encyclopædia Britannica says:
A cavetto is a concave moulding with a regular curved profile that is part of a circle, widely used in architecture as well as furniture, picture frames, metalwork and other decorative arts. In describing vessels and similar shapes in pottery, metalwork and related fields, "cavetto" may be used of a variety of concave curves running round objects. The word comes from Italian, as a diminutive of cave, from the Latin for "hollow" (it is the same root as cave). A vernacular alternative is "cove", most often used where interior walls curve at the top to make a transition to the roof, or for "upside down" cavettos at the bases of elements.
The cavetto moulding is the opposite of the convex, bulging, ovolo, which is equally common in the tradition of Western classical architecture. Both bring the surface forward and are often combined with other elements of moulding. Usually, they include a curve through about a quarter-circle (90°). A concave moulding of about a full semi-circle is known as a "scotia".
Egg-and-dart, also known as egg-and-tongue, egg-and-anchor, or egg-and-star, is an ornamental device adorning the fundamental quarter-round, convex ovolo profile of moulding, consisting of alternating details on the face of the ovolo—typically an egg-shaped object alternating with a V-shaped element (e.g., an arrow, anchor, or dart). The device is carved or otherwise fashioned into ovolos composed of wood, stone, plaster, or other materials.
Egg-and-dart enrichment of the ovolo molding of the Ionic capital was used by ancient Greek builders, so it is found in ancient Greek architecture (e.g., the Erechtheion at the Acropolis of Athens), was used later by the Romans and continues to adorn capitals of modern buildings built in Classical styles (e.g., the Ionic capitals of the Jefferson Memorial in Washington, D.C., or the ones of the Romanian Athenaeum from Bucharest). Its ovoid shape (the egg) and serrated leaf (the dart) are believed to represent the opium poppy and its leaves. The moulding design element continues in use in neoclassical architecture. As a mass-produced architectural motif at the turn of the 19th to the 20th century, it can, when seen alongside dentils (tooth-like blocks of wood in rows), be used to date a building to the Edwardian period, which began with the death of Queen Victoria in 1901.
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