A square head bolt has a four-sided head.
Square Head Bolt, Steel, Grade 2, Zinc Plated, 5/16"-18 Thread Size, 1-1/2" Lg, 10 PK. Square bolts are typically mated with square-headed nuts.Compared to standard hex bolts, square bolts have a greater surface in contact with the part being fastened, and therefore provide greater resistance to loosening (though also greater resistance to tightening).