1/2"-13 x 2-1/2" 18-8 Stainless Steel Coarse Thread Carriage Bolts 4PK, Midwest Fastener, Mfr#: 78924

A carriage bolt has a domed or countersunk head, and the shank is topped by a short square section under the head. The square section grips into the part being fixed (typically wood), preventing the bolt from turning when the nut is tightened.