Saussure emphasizes the differential quality of the sign, comparing the value of the sign to the value of a chess piece. If a knight is lost, another object can be substituted for the knight and assume the value of the knight, with the agreement of both players. The characteristics of the object are not important. Once it is substituted for the knight, its difference from the other pieces allows it to assume the unique value which the knight enjoys according to the fixed rules which govern chess. Similarly, a linguistic sign is accorded value within the system of rules which govern language because of its difference from other signs.
Syntagmatic relations occur in discourse, as a result of the linear nature of language.
A syntagm is a combination of terms which appear in sequence and acquire value through their difference from each other. Associative relations involve the non-discursive mental operation through which memory aligns terms according to various categories. This is essentially what a psychotherapist relies upon for the success of word-association techniques. The patient's memory organizes words into associative categories which may provide clues about traumatic experiences. Since mutability rules in the realm of diachrony, the identity of the linguistic unit is an interesting problem.