I pulled a spiral shank nail out of ironbark once. Once. The board splintered in three directions, the nail head tore through, and I spent the next ten minutes with vise grips getting the rest of the shank out. My mate in Brisbane just laughed. Told me he'd rather pull his own teeth. Spiral shank nails — screw shank in some catalogs, same thing — they don't look like trouble in the box. Smooth head. Slim body. Roll one between your fingers, though, and you feel it right there: a helical ridge, unbroken, top to bottom. Like a wood screw with no slot. That ridge changes how the nail goes in, how it stays in, and whether your work survives the next cyclone season. If you're speccing fasteners for anything that moves, breathes, or sits outside, you need to know what these things actually do.