Final answer:
The flat upper member of a capital is known as the abacus, serving as a crucial structural and decorative element in classical architecture.
Step-by-step explanation:
The flat upper member of a capital is called the abacus. This component of a column's capital is crucial in classical architecture as it provides a flat surface which supports the architrave or an arch above. Especially in ancient Greek and Roman architecture, the abacus can be found in various shapes, usually square or circular, and is often found atop the sculptured foliage or volutes in the capital designs.
For example, a capital carved with a stylized version of an acanthus-leaf design from the late 8th century showcases the refined skills of artisans at the time. This can be seen in artworks such as the capital with leaves from Syria, part of The Metropolitan Museum of Art collection. These designs were not just decorative, as capitals and their design elements, like the abacus, played a key structural role.