Final answer:
In the context of architectural elements, the rectangular impost block that transmits the weight of an arch between the capital of a column and the springing of an arch is called an Impost. It should not be confused with Keystone, Corbel or Abutment, which have distinct functions in the structure and geometry of an arch.
Step-by-step explanation:
The protruding molding at the base of an arch in a pier, which involves a rectangular impost block that transmits the load of the arch to a supporting mechanism possibly situated between the capital of a column and the base of the arch, is called an Impost. Basically, an Impost is an architectural feature which allows the weight of the superstructure to be transferred laterally, allowing the columns or piers to support the structure from underneath. This differs from a Keystone, which is the wedge-shaped stone piece at the top of the arch, and a Corbel, which refers to a type of arch constructed with successive courses of stone projecting towards the archway's center. An Abutment serves to resist the lateral forces pushing an arch outward.
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