What type of siding consists of two pieces of stock placed vertically on the corners?

The correct answer pertains to the distinct configuration and function of corner boards in siding construction. Corner boards are specifically designed as vertical strips of wood or other materials that are installed on the exterior corners of a building. They serve both a structural and aesthetic purpose by covering the seams where two wall panels come together, ensuring a clean transition at the corners. This not only reinforces the corners but also provides a finished appearance that enhances the overall look of the siding.

In contrast, other types of siding mentioned have different characteristics and applications. Panel siding involves large flat panels usually applied horizontally, clapboard siding consists of narrow, overlapping boards installed horizontally that create a weather-tight barrier, and horizontal siding refers to any siding that is laid out across a structure in a horizontal line rather than vertically. Each of these alternatives serves its own purpose and design principle, but they do not share the same structural role as corner boards.