What are the main features of Ferdinand de Saussure's sign theory?

The first important feature of Saussure's linguistic theory is his participation in a shift from the primarily diachronic studies of earlier comparative philologists to synchronic studies. Second, a feature that was enormously influential on subsequent literary theorists was his interest in the primacy of spoken rather than written language, something facilitated by the technological innovation of recording the human voice (phonographs, tapes) which made it possible for linguists to freeze and examine speech. Next, in his analysis of speech he rejected earlier concepts of a narrowly referential account of meaning and translatability, instead emphasizing the nature of sign as langue and parole existing as a system of differences within a complete linguistic system. He also emphasizes the arbitrary nature of the relationship between signifier and signified with a sign.

What are Saussure's fundamental assumptions of "SIGN"?

While it is easy to jump over Saussure's, shall we say, pure sign theory and go straight to applications or theories that grew out Saussure's discussion of sign, the four fundamentals of sign as posited by Saussure are still the critical underpinnings. These four fundamental assumptions of sign posited by Saussure, as spelled out by Columbia University's Media Center, are that sign is: diadic, arbitrary, relational, differential.

Sign Is Diadic: That sign is diadic means it is based on two: it has two inseparable parts or components. Sign may not be considered as a singular concept. The diadic parts are the signifier and that which is signified: signifier/signified. These are inseparable in experience. If we use the signifier "couch," there is a signified object or concept that is inseparably associated with it. At the same time, "couch" refers to multiple signifieds, such as your idea of Country plaid and my idea of Contemporary stuffed leather.

Sign Is Arbitrary: Signs developed through the history of language by convention since there is no innate, no natural relationship between signifier and what it signifies. For instance, there is no natural, innate relationship between "horse" and the large running beast with soulful eyes and a swishing tail. It was convention that designated this animal as "horse." This assignment by convention is a result of the arbitrary, non-natural, nature of sign. This arbitrary quality of sign leading to conventional assignment of signifier/signified applies to images as well as to language, thus the field of semiotics could be developed.

Sign Is Relational: This means that since sign is arbitrary and thus conventional (assigned by convention, or agreement within a society), when sign (sign: signifier/signified) is in a system of signs, it gains meaning from the relationship between them, just like each sign in this sentence gains meaning from each other sign. In other words, the parts work together to give meaning to each other because of how they relate to each other. This description of the relational quality of sign applies equally to language and to images.

Sign Is Differential: While sign derives meaning from relations with other signs in the same system, sign defines or differentiates by what it is not. As an example, "Dog is" derives meaning through relationship between signs while at the same time defining by differentiating from "Cat is," which it is not. A more complicated example involves both intentional and accidental ambiguity (TU Dresden) created by language and images that may be one of two things depending upon how understood or seen. Thus sign defines by differentiating while giving meaning by relation.

sing is diadic--comprised of two parts: the signifier and the signified

sign is arbitrary--there is no natural reason why a signifier is linked to a signified

sign is relational--sign only makes sense in relation to other signs in the same system

sign is differential--sign defines things by what they are not rather than by what they are (Columbia University, Media Center)

What are Saussure's fundamental assumptions of "SIGN"?

In his Cours de linguistique générale, or Course of General Linguistics, Ferdinand de Saussure (1916) aimed to provide the field of philology with a comprehensive breakdown s to what language is, what it is made of, how it is put together, and with what intentions and differences it is used.

The historical context of the Cours is significant, as it represents a major milestone never before considered in the study of linguistics. Some of the assumptions may sound redundant to the modern student. But the foundations were studied as closely before.

Out of Le Cours came "semiotics", or the study of the signs and the symbols that make up our communicative language.

This Saussurean concept ultimately focused in fundamental assumptions such as the following.

• Language and Speech are two separate concepts- While language is a system of signs, "speech" is a combination of utterances.
• Signs, as the building blocks of language, are what we know as "words".
• The job of the "signs" is to connect the visual symbols of the sounds that we hear. Hence, the assumption is that, as we hear sounds, we create a mental image.
• The signs (words), also made up of basic elements (phonemes/letters), represent themselves separately from the image that we create. Saussure calls this the "signifier".
• The image created as a result of exposure to the sounds and signs, is referred by Saussure as the "signified".
• The sign is arbitrary. This means that it has no connection with the signified. In Saussure's own (often quite complex) words,

The term "arbitrary" should not imply that the choice of the signifier is left entirely to the speaker…; I mean that it is unmotivated, i.e. arbitrary in that it actually has no natural connection with the signified.

• The spoken word is, as Saussure says "phonocentrally privileged", as it produced acoustic images, "des images acoustiques" , as he formally terms them. 

Hence, these are the basic assumptions that can be extrapolated from Saussure's lesson on SIGNS as they relate to language. In a more casual way, what Saussure aims to explain is that we can control language, but the effect and imagery that language produces is out of our reach. The signifier (what you say), and the signified (what is understood) are separate. In fact, the signified is entirely dependent on social interaction. Why? Because without schema we cannot produce the mental imagery that comes as a result of hearing speech.

The arbitrary nature of the sign explains why the social fact alone can create a linguistic system. The community is necessary if values that owe their existence solely to usage and general acceptance are to be set up; by himself, the individual is incapable of fixing a single value

Think about what happens when you hear a foreign language that you do not know. What sort of mental imagery can you make upon merely hearing the utterances of foreign speech? None whatsoever, because you have not built any prior knowledge (schema) upon which you can build a concept. However, if you live in that foreign country long enough you may become familiar with the signs of that language in the form of commercial prints, graffiti, the billboards announcing products, restaurant menus, and others. When the entire sign is put together (sound and symbol), it becomes the signifier.  Only after you put the two together, and interact with the signifier is that you can produce what is signified in your mind.