These Precision(R) grade 2 round head carriage bolts are made of zinc-plated steel. They have a thread of 3/8"-16 and a length of 6". Sold in boxes of 20 bolts, they come without nuts. This type of bolt is primarily used with wood. The square that is found under their head pulls into the wood as the nut is tightened. Their zinc plating grants them a moderate protection against corrosion.