 The fundamental notion with reference to which the different syntactical and semantic rules of systems can be explained is that of a notation—in brief, a symbol system where to each symbol corresponds one item in the realm, and to each item in the realm only one symbol in the system. Hence, for instance, a musical score is a character in a notational system only if it determines which performances belong to the work and, at the same time, is determined by each of those performances (Goodman 1976, 128–130). In a notational symbol scheme all the members of a character are interchangeable (that is, there is “character-indifference” between the marks that make a character) (Goodman 1976, 132–134). Hence, for instance, the Roman alphabet is made of characters in a notational scheme because any inscription of, say, the letter “a” (A or a or a…) expresses the same character and hence can be chosen at will, and because each of such marks cannot be used for any other letter of the alphabet. The same is true, for instance, of the set of the basic musical symbols used in standard musical notation. Accordingly, the two syntactic requirements of a notation are disjointness (each mark belongs to no more than one character) and finite differentiation, or articulation (in principle, it is always possible to determine to which character a mark belongs). Symbol schemes that are notational can be compared in their workings to the way digital instruments of measurement work: for any measurement indicated by the instrument there is always a definite answer to the question, What is the measurement? By contrast, schemes that are non-notational are well exemplified by analog systems of measurement. For their complete lack of articulation, those systems can also be said to be dense throughout: given any mark (e.g., a mark in a scale) it could stand for virtually an infinite number of characters, hence of measurements; or, equivalently, given any two marks, there is a virtually infinite number of possible characters between them. 