OK. There are flat head screws and Phillips head screws and Robertson head screws. The shank can be tapered or straight. It can have a deep thread or shallow. There are metal screw... moreOK. There are flat head screws and Phillips head screws and Robertson head screws. The shank can be tapered or straight. It can have a deep thread or shallow. There are metal screws and wood screws and sheet metal or self-tapping screws and machine screws and concrete or masonry screws. It is mind-boggling. Do you have to go to school to learn which screw to use when? My dad had a few different sizes in his workshop. Jim has a few types in his workshop. But thousands of sizes? Geez, why thousands? less