At the heart of the semiotic enterprise are systems of conventional signs used for direct communication. These include, first, the various codes used to convey messages composed in an existing natural language, such as English. Morse code, semaphore codes, braille, and all the codes devised for secrecy can be used to convey an English message. Second, there is a whole series of specialized codes used to convey a particular type of information to groups who may not share the same natural language: chemical symbols, traffic signals and road signs, silver assay marks, mathematical symbols, the signs used in airports, trains, etc., and finally the recondite symbolisms of heraldic or alchemical codes. All these cases involve conventional signs based on explicit codes: since they are designed for easy and unambiguous communication, there is an explicit procedure for encoding and decoding, such as looking up the item in question in a codebook. Such codes are pure examples of semiotic systems, but precisely because they are so straightforward it is usually an easy matter to describe the principles on which they are constructed, and so they often prove much less interesting than less explicit and more complicated systems which fall into our next category.
More complicated than explicit codes are systems were communication undoubtedly takes place, but where the codes on which the communication depends are difficult to establish and highly ambiguous or openended. Such is the case, for example, with literature. To read and understand literature, one requires more than a knowledge of the language in which it is written; but it is very difficult to establish precisely whatsupplementary knowledge is required. Certainly, one is not dealing with the sorts of codes for which the keys or codebooks could be supplied. However, precisely because one is dealing with an extremely rich and complicated communicative system, the semiotic study of literature and other aesthetic codes (such as those of painting and music) can be extraordinarily interesting.
The reason for the evasive complexity of these codes is quite simple. Codes of the first type are designed to communicate directly and unambiguously messages and notions which are already known; the code simply provides an economical notation for notions that are already defined. But aesthetic expression aims to communicate notions, subtleties, complexities that have not yet been formulated, and, therefore, as soon as an aesthetic code comes to be generally perceived as a code (as a way of expressing notions which have already been articulated), then works of art tend to move beyond it. They question, parody, and generally undermine it, while exploring its possible mutations and extensions. One might even say that much of the interest of works of art lies in the ways in which they explore and modify the codes they seem to be using; this makes semiotic investigation of these systems both highly relevant and extremely difficult. (116-7)
Culler, Jonathan. Ferdinand de Saussure. Cornell University Press 1991.