What is the size relationship between actual and nominal dimensions of a block?

The correct understanding of the relationship between actual and nominal dimensions of a block lies in the definition of each term. Nominal dimensions refer to the dimensions of a masonry unit as specified by the manufacturer, which typically include additional space for mortar joints when these units are laid in a wall. This means that the nominal dimensions are larger because they incorporate the expected thickness of the mortar that will be used during installation.

In contrast, actual dimensions are the physical size of the block itself, excluding any mortar that may be applied during construction. Therefore, when blocks are laid with mortar, the overall wall thickness is effectively increased by the mortar joint, which is why nominal dimensions are larger—because they account for the unit and the mortar that will be applied. This concept is crucial for determining how many blocks are needed for a project and how they will fit together, ensuring proper spacing and aesthetic alignment in masonry work.

This clarifies why the answer highlights the importance of understanding both dimensions in practical masonry applications, especially regarding how walls are constructed and designed.