Similar to a wood screw except that it is generally much larger running to lengths up to 15 in (381 mm) with diameters from 0.25-0.5 in (6.35-12.70 mm) in commonly available sizes and it generally has a hexagonal drive head. Coach Screws are designed for securely fastening heavy timbers (post and beams, timber railway trestles and bridges) to one another, or to fasten wood to masonry or concrete.