Saussure's dyadic model views the sign as consisting of the signifier and the signified, where the relationship between the two is arbitrary. Pierce expanded on this with his triadic model, which includes an interpretant component. The interpretant is how a sign is understood or interpreted. Umberto Eco categorized signs into natural and artificial signs. Natural signs are not intentionally produced, while artificial signs are. He further breaks down artificial signs based on their intended function. Logical codes are more static and have a single meaning, while social and aesthetic codes are more fluid and open to various interpretations.