Single meaning they are centrally aligned and don't lean to one side more than the other. In other words, there's no predominant right or left side they are perfectly balanced. They aren't part of a set and they don't have any arrowheads that are designed to be paired with them, though they can be mixed with other arrowheads if desired. They are designed to stand on their own and still look good. They are single and work best on their own.