It is Thridness in the First Correlate. In Peirce’s definition, it is a sign that is a law, rule or convention (not necessarily a product of the human mind, though). It can be a concept in the mind of a human being but also a habit that regulates conventional behaviors of many living species. A Legisign is a general, a universal that acts in the reality, a would be whose strength is never depleted by any finite series of its individual instances.