Final answer:
The vertical guideline through the eye of a volute in an Ionic capital is known as the cathetus. This structural feature is crucial for determining the shape of the ornamental spirals characteristic of the Ionic order, setting it apart from other columnar styles like the Doric and Corinthian orders.
Step-by-step explanation:
The vertical guideline through the eye of a volute in an Ionic capital, from which the spiral form is determined, is known as the cathetus. Volutes are ornamental features shaped like scrolls or cinnamon rolls and are a defining characteristic of Ionic capitals. Unlike the simple and rather austere capitals of Doric columns, which feature a square abacus and a flared echinus, the Ionic capitals are more elaborate with their scroll-like volutes.
The volutes ornaments are important in distinguishing the Ionic order from other orders such as Doric and Corinthian. The Corinthian order, for instance, is known for its elaborate capitals with acanthus leaves, while the Doric order is characterized by more simplistic and sturdy geometric forms.
The Ionic order originated in the region of Ionia and became widespread in mainland Greece by the fifth century BCE. Detailed carvings, such as those on the Ionic capital north porch of the Erechtheion, demonstrate the intricacy and skill required to create these spiral volutes that depend upon the precise placement of the cathetus for their elegant form, a concept that is illustrated in Figures 9.1.12 and 9.1.13 from Smarthistory. This system added a distinctive aesthetic to the columns, contributing richly to the visual language of classical architecture.