ASME B18.5 is the governing American standard for Round Head Square Neck Bolts (Inch Series). These fasteners, universally known as "Carriage Bolts," are defined by their smooth, dome-shaped head and a square neck designed to sink into wood or a square hole in metal to prevent rotation while a nut is tightened.

This standard is critical for North American manufacturing, spanning from agricultural machinery and playground structures to high-end furniture and residential construction. Because the head has no external drive (hex, slot, or socket), it offers a clean aesthetic and enhanced security, as the bolt cannot be easily removed from the head side once installed.

The technical focus of ASME B18.5 is clamping efficiency in non-metallic joints. The "Round Head" design is specifically engineered to provide a large bearing surface area ($H$), which prevents the head from marring or embedding too deeply into softer substrates like pressure-treated lumber. The "Square Neck" ($P$) acts as the anti-rotation mechanism; it is essential that the square neck is fully seated to ensure that the assembly can achieve the required tension without stripping the pilot hole.