The square shoulder on carriage bolts help to hold it in place while the nut is fastened. Carriage Bolts 5/16in-18 X 4in A307 Grade A Zinc Plated Steel 50PK Size: 5/16 inch-18 X 4 inchFeatures a smooth rounded head as opposed to a standard hex headSquare shoulder allows for nut to be tightened or loosened Often used in cases where a protruding bolt head could cause injuryNuts and washers not included, ensure washers and nuts are same diameterThread Type: Coarse, Thread Class: UNC-2A, Point Style: BluntSpecification(s) Met: ASME B18.5 & ASTM A307A.