Abstract: Within the framework of the Theory of Descriptions, the semantic meaning of the structure "NP1 is NP2" is determined by the attributes of the two NPs together. When NP1 is a description, while NP2 is a name, the structure expresses the relationship of specificational reference. On the contrary, when NP1 is a name, NP2 is a description; the structure expresses the relationship of attribution. When both NP1 and NP2 are names, the structure expresses the meaning of equivalence. When both NP1 and NP2 are descriptions, the structure expresses the meaning of equivalence or attribution. NP1 and NP2 can exchange their positions without altering the meaning of the construction only when it expresses the meaning of equivalence.