Brass Square bolts (also known as square head bolts or square head machine bolts) offer less danger of wrench slippage in hand wrenching, as well as a greater bearing area. In addition, the square head style is more secure than a hex bolt style when fixed into a cored recess. Square bolts are now most commonly used for aesthetic purposes to provide a rustic look in a new structure or to match existing fasteners in an older structure.