You may not realize it, but metal has a grain. Like wood, a metal’s grain affects its properties. The grain comes from the fact that most sheetmetal starts life as a round bar that is hot rolled or cold rolled into sheets or plates of different thicknesses. So the grain (usually somewhat visible) runs parallel to the long axis of the original metal bar. It’s generally best to bend metal perpendicular to the grain (90 degrees to it). Bending along the grain can lead to cracking and splitting of the material, or at least a weaker piece of metal.