Enu-kugi, Shienu-kugi (N and CN nails; エヌ釘、シーエヌ釘) – specialized color-coded nails with lengths printed on the heads, required by law for certain structural applications here in Japan. These nails have thicker diameters than normal nails of the same length, and are used for structural diaphragms such as walls, floors, and roofs at a pre-determined pitch — typically between 75 and 200 millimeters.
The image below shows the conventional N-nails used for standard wooden house-building.
This image shows what’s called CN-nails, and it’s what’s used for 2 by 4 houses — or the 2 by 4 house construction method. 2 by 4 houses don’t use diagonal bracing and derive all their shear strength from their diaphragms. They therefore have higher pitch requirements.