 Leibniz’s Law has itself been subject to controversy in the sense that the correct explanation of apparent counter-examples has been debated. Leibniz’s Law must be clearly distinguished from the substitutivity principle, that if “a” and “b” are codesignators (if “a=b” is a true sentence of English) they are everywhere substitutable salva veritate. This principle is trivially false. “Hesperus” contains eight letters, “Phosphorus” contains ten, but Hesperus (the Evening Star) is Phosphorus (the Morning Star). Again, despite the identity, it is informative to be told that Hesperus is Phosphorus, but not to be told that Hesperus is Hesperus (“On Sense and Reference” in Frege 1969). Giorgione was so-called because of his size, Barbarelli was not, but Giorgione was Barbarelli (Quine, “Reference and Modality”, in 1963) . It is a necessary truth that 9 is greater than 7, it is not a necessary truth that the number of planets is greater than 7, although 9 is the number of planets. The explanation of the failure of the substitutivity principle can differ from case to case. In the first example, it is plausible to say that “‘Hesperus’ contains eight letters” is not about Hesperus, but about the name, and the same is true, mutatis mutandis, of “‘Phosphorus’ contains ten letters”. Thus the names do not have the same referents in the identity statement and the predications. In the Giorgione/Barbarelli example this seems less plausible. Here the correct explanation is plausibly that “is so-called because of his size” expresses different properties depending on the name it is attached to, and so expresses the property of being called “Barbarelli” because of his size when attached to “Barbarelli” and being called “Giorgione” because of his size when attached to “Giorgione”. It is more controversial how to explain the Hesperus/Phosphorus and 9/the number of planets examples. Frege’s own explanation of the former was to assimilate it to the “Hesperus”/“Phosphorus” case: in “It is informative to be told that Hesperus is Phosphorus” the names do not stand for their customary referent but for their senses. A Fregean explanation of the 9/number of planets example may also be offered: “it is necessary that” creates a context in which numerical designators stand for senses rather than numbers.