A round head bolt with a square neck under the head, designed to keep the bolt from turning as a nut is tightened.
The most common alloy of copper, brass is basically 2/3rds copper, 1/3 zinc. It is non-magnetic with good strength and toughness, high electrical conductivity, and an attractive lustrous finish. It has good corrosion resistance, but not in salt water. Brass is commonly used by the electrical and communications industries, builders hardware, and some marine applications.