A carriage bolt, also known as a coach bolt or round head square neck bolt, is a type of fastener used to secure two pieces of wood or metal together. Carriage bolts have a large, domed or rounded head. The head may have a square neck or ribbed shoulder just below it, which is designed to fit into a pre-drilled hole to prevent the bolt from spinning when tightening the nut. The threaded portion of a carriage bolt extends from the head to the end of the shank. The thread can be coarse or fine, depending on the specific application.The square shoulder beneath the head is an essential feature of a carriage bolt. It fits into a square hole or recess in the material being fastened, preventing the bolt from rotating during installation or removal.
Carriage bolts are commonly made from steel, stainless steel, or other metals. They are available in various grades, including low carbon (Grade 2 or 4.6), medium carbon (Grade 5 or 8.8), and high carbon alloy (Grade 8 or 10.9), each with different strength properties.Carriage bolts come in a range of sizes, typically specified by their diameter, length, and thread pitch. Common diameters include 1/4", 5/16", 3/8", 1/2", 5/8", and 3/4".
Carriage bolts are commonly used in woodworking and construction projects where a secure joint is required. They are often used for fastening metal brackets, hinges, and other hardware to wood or metal structures. Carriage bolts are also used in assembly and installation of machinery, fences, decks, and outdoor furniture.

To install a carriage bolt, a hole is drilled into the material being fastened, with a diameter slightly larger than the bolt's shank. The square hole or recess for the shoulder is then created using a chisel or square hole punch. The bolt is inserted through the hole, and a washer and nut are threaded onto the protruding end of the bolt. The nut is then tightened to secure the joint.