The rivet family is generally divided into six genuses: fully tubular, semi-tubular, self-piercing, split, tapped & compression rivets. Depending on their niche & matrix, they may be made up of copper, brass, aluminum, stainless steel or carbon steel, and their heads may be flat, oval-shaped, counter-sunk or trussed. Fully tubular rivets are mostly hollow, with a hole depth equal to or greater than 112 percent of the diameter of the body, while semi-tubular rivets, the most commonly encountered genus, have a hole depth less than 112 percent of the diameter of the body. It’s unclear, however, to what extent this classification reflects a meaningful cladistic distinction. Self-piercing rivets, despite their name, do not pierce themselves, but simply pierce sheet metal or aluminum by themselves, without needing to fit into pre-existing holes. Split rivets have evolved to inhabit soft materials—wood, light metals, leather & fibers—which they grip in two ways, the body piercing the material & the sharp prong ends folding back and biting in. Tapped rivets are found in materials too thin to accept their own tapping—a mutualistic arrangement. Compression or cutlery rivets, with their solid bodies & chamfered shanks, have adapted to the extreme environments found in the handles of knives.