A layer is the shortest unit of a design that is built in AM. It is usually defined by its thickness, shape, and area. The definition needs to be revisited. When a design is divided into a number of layers, a layer is not meant to be different from its projection on a horizontal plane. If the design is simple, e.g., a rectangular block, the layer of any thickness gives the same projection, which does not create the difference between a layer of any thickness and its projection. If the design is not such a simple one, e.g., a sphere, the layer of any thickness does not give the same projection. For such a design, if a thick layer is selected, it gives problems because it has many projections. If the differences in projections are ignored and the layer is selected to manufacture a sphere, the result will not be a sphere. Therefore, a layer of any thickness can be selected as long as the selection does not create more than one projection.