Twin Tip meaning they are each a set of two arrowheads identical but mirrored from one another. In other words, there's a right and left side or a top and bottom for each arrowhead style. They aren't part of a set, though they can be mixed with other arrowheads if desired. They're designed to be used together to create a consistent and balanced symmetrical look. They're twins so they look alike and work together side by side.