Carriage bolts have a rounded low-profile smooth head with no drive that sits on a flat surface and a square shank immediately underneath the rounded head that sinks into the material being fastened to secure the bolt from turning after installation with threads running further down the neck sometimes using nuts and washers. Carriage bolts are coarse thread (UNC) bolts used for applications such as safety and security, wood to metal fastening, wood to wood fastening and other industrial uses and are sometimes referred to as carriage screws and are available in various materials such as zinc plated, stainless steel, and hot dipped galvanized.
