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There are many models of the linguistic sign (see also sign (semiotics)). A classic model is the one by the Swiss linguist Ferdinand de Saussure. According to him, language is made up of signs and every sign has two sides (like a coin or a sheet of paper, both sides of which are inseparable):
the signified (French signifié), the ideational component, the concept or object that appears in our minds when we hear or read the signifier e.g. a small domesticated feline (The signified is not to be confused with the "referent". The former is a "mental concept", the latter the "actual object" in the world)
Saussure's understanding of sign is called the two-side model of sign.
Furthermore, Saussure separated speech acts (la parole) from the system of a language (la langue). Parole was the free will of the individual, whereas langue was regulated by the group, albeit unknowingly.
Saussure also postulated that once the convention is established, it is very difficult to change, which enables languages to remain both static, through a set vocabulary determined by conventions, and to grow, as new terms are needed to deal with situations and technologies not covered by the old.
The Concept of Arbitrariness 
According to Saussure, the relation between the signifier and the signified is "arbitrary", i.e. there is no direct connection between the shape and the concept (cf. Bussmann 1996: 434). For instance, there is no reason why the letters C-A-T (or the sound of these phonemes) produce exactly the image of the small, domesticated animal with fur, four legs and a tail in our minds. It is a result of "convention": speakers of the same language group have agreed (and learned) that these letters or sounds evoke a certain image.
Compare an aerial drawing of London (field of potential signifieds) with a grid (field of signifiers) placed on it. The grid is arbitrary. Its structure (however motivated) divides the drawing into areas (which can then be referred to). The division of the drawing is arbitrary. A square 'EC1' is an inseparable fusion of grid and area of drawing i.e. is a sign - just like two sides of the same sheet of paper - which refers to 'real' land. EC1 does not have to refer to the particular part of London it does. Drawing + grid = map = language.
Two concepts are often cited to disprove Saussure’s claim, however, he provides reasons as to why these concepts are irrelevant. They are:
Which applies only in a very limited number of cases, and stems from phonetic approximation of sounds, which can themselves evolve into a more standard linguistic sign, and
Which fall much to the same logic as onomatopoeia, as is demonstrated by comparisons of the same expression in two languages (e.g. the French aïe and the English ouch).
Likewise, the figures made in writing are arbitrary, and not connected to the sounds which they inspire. The only requirement is the ability to differentiate between separate figures, such as t, l and f, and that the difference in the symbols is understood by the collective consciousness (i.e. that "i" is recognized as "i" by all members of the community, no matter what word it is placed in).
Criticisms of Saussure 
Saussure’s theory has been criticised, for instance for confusing words as sound-patterns with words as signs. As Marya Mazor states, “It does not make sense to say that a word can be exchanged with an idea if, as a sign, such an idea is part of its makeup.” She goes on to point out that in the exchange of words, Saussure views words as signs, as Mazor calls it, “meaning-and-form combinations,” leading to a rejection of real-world context. In viewing words as the “coins” of the language, Saussure sees them as interchangeable with other words or ideas-a viewing of words as sound-patterns. However, in word exchange, the word is contextually defined, and the exchange of another word “coin” in its place will never be precise; in short, it is an inexact trade (Mazor, 7).
Other Viewpoints 
Rudi Keller gives a simplified definition of linguistic signs, if not signs in general, stating
"Signs, therefore, are clues with which the speaker “furnishes” the addressees, enabling them and leading them to infer the way in which the speaker intends to influence them. Signs are not […] containers used for the transport of ideas from one person’s head to another. Signs are hints of a more or less distinct nature, inviting the other to make certain inferences and enabling the other to reach them." (90)
Keller dubs the process of making inferences “interpretation,” and the goal of the process “understanding” (90).
Michel Foucault proposed a linkage between linguistic signs and their cultures, stating that language practices help to maintain assumptions in a culture by serving as a tool for knowing and constructing the world. He calls this connection between the physical reality and the discursive reality the “dominant discourse” and gives the example of “freedom” in the United States. The “freedom” stressed in the U.S. places emphasis on the individual, unhampered, and this viewpoint persists despite workplaces that require subordination and laws that refine freedom’s limits. “Freedom” in the U.S. persists in being defined as such, despite physical realities to the contrary (Rivkin, 54)
In addition to discussing and refining the concepts, offered by de Saussure, there appeared different models of sign. For example, Gottlob Frege in logics and philosophy, as well as C.K. Ogden and I.A. Richards in semiotics, offered a three-side model of sign named the triangle of reference. This model looks like a triangle that unites three points: symbol, referent (object), thought of reference, while the sides of the triangle depict the relationships between them.
See also 
- Bussmann, Hadumod (1996), Routledge Dictionary of Language and Linguistics, London: Routledge.
- Saussure, Ferdinand de (1916), "Nature of the Linguistics Sign", in: Charles Bally & Albert Sechehaye (Ed.), Cours de linguistique générale, McGraw Hill Education. ISBN 0-07-016524-6.
- Keller, Rudi. “Expression and Meaning.” A Theory of Linguistic Signs. By Rudi Keller. Trans. Kimberley Duenwald. New York: Oxford University Press, 1995. 87-95.
- Mazor, Marya. “Really Relativism: Dialectic Interpretations of Saussure.” Structuralism and Post-Structuralism. Ed. Imtiaz S Hasnain. New Delhi: The Authors and Bahri Publications, 1992. 1-17.
- Rivkin, Julie, and Michael Ryan. “The Implied Order: Structuralism.” Structuralism, Linguistics, Narratology. 53-55.
- C. K. Ogden and I. A. Richards (1923), The Meaning of Meaning, London and New York.
Web References 
http://signelinguistique.e-monsite.com/ For a promotion of unconscious motivation of words