A linguistic expression may be motivated or arbitrary. This concerns the relationship of the significans to the significatum. The significans and its structure may be partly motivated by the significatum and its structure. This entails that speaker and hearer do not, to this extent, need to know the particular language system to form the sign, as its significans is partly predictable on the basis of its significatum, and vice versa.

Again, the relationship between significans and significatum may be partly arbitrary. This means that the significans has properties which are not predictable on the basis of the significatum, and vice versa. Instead, they derive from constraints of the particular language system which concern the formation of signs in this language.

Motivation and arbitrariness are relevant at two different levels of the formation of linguistic signs. At the lower level, the significans of a simple sign (a morpheme or root) is combined with its significatum. This association is arbitrary for most simple signs. Only to some extent does motivation come in by onomatopoeia and sound symbolism. At the higher level, a (presumably complex) meaning is coded by a complex sign, i.e., a morphological or syntactic construction. Here the hierarchy of complexity levels of linguistic structure comes in:

Constraints of the language system are the stricter, the lower the level of structure. Specifically, at the highest level, few operations are specific to the language used and subject to its constraints. At the lowest level of structure, i.e. the morphological level, obligatoriness prevails, i.e. the formation of units is largely dictated by language-specific constraints. Again, the constraints at the highest level are relatively transparent to motivation, while the constraints at the lowest level may be completely arbitrary.

For illustration, imagine a situation where the speaker has solved the task of associating significans with significatum at the lowest level of the preceding diagram; i.e., he knows the elementary words of the language he must speak. That is all he knows; he does not know a single rule of its grammar. In an emergency situation, he may produce a.

Observe that a may be completely appropriate in the speech situation and may be perfectly intelligible. What the speaker has done is apply operations of the uppermost level of the hierarchy of complexity levels to the vocables he knows. His utterance does have a structure; but it reflects the structure of his thought without regard to any linguistic constraints.

Now compare with this the English version in b. The following of its features derive from constraints specific to this language:

• At the level of complex sentence formation, there is a rule of pronominalization. It is not obligatory, but contributes to coherence of the message. The pronoun agrees with its antecedent.
• At the level of the simple clause, some rules of syntax obtain. A state like ‘sick’ is coded by an adjective. However, to predicate an adjective on a subject requires a copula. The subject of such a predication goes at the start of the clause by default. The same goes for the second clause. The predicate of this clause is, however, a verb. In the present tense, it obligatorily agrees with a third person singular subject. Its second participant is its direct object.
• At the level of the syntagma, most rules are even stricter. In such a context, a kinship term like daughter requires a possessive attribute or determiner; thus an item like my is obligatory, and it must precede the noun designating the possessed item. Likewise a singular noun heading a noun phrase must be preceded by a determiner; thus some formative like a in front of the noun physician is obligatory. In a predicate phrase consisting of a copula and an adjective, the former must precede the latter. Likewise in a verb phrase consisting of a verb and its direct object, the former must precede the latter. Since the adjective urgent modifies a verbal construction, it must be adverbialized.
• At the level of the word form, the following constraints obtain: Given the agreement of the anaphoric pronoun, its form shows gender and number. Given the agreement of the copula and the verb, the form of the former is a member of a suppletive paradigm, the latter bears a conjugation suffix. Given the adverbialization of the adjective urgent, adverbialization is by a suffix.

The following aspects of this example instantiate general principles:

• The grammatical constraints are language-specific. Other languages would apply different constraints or may apply no constraint at all in the given context. Thus, a Latin version of would normally lack the possessive determiner in front of ‘daughter’; the fact that she is someone's daughter follows from the semantic relationality of this noun, and the fact that she is the speaker's daughter is inferrable from the speech situation. Many languages including Mandarin and Russian would not require a copula in the first clause. Other languages, including Cabecar (Chibchan), would use a copula and a verb in these two clauses, but would lack any rule of agreement of these words with their subject. The order of subject, verb and object in the second clause may just be any, depending on the language. Few languages, among them Latin, would require the adjective in the second clause to be adverbialized, while Mandarin, Cabecar and German are not among these languages.
  

  Thus the true difference between languages is not in what may or may not be expressed but in what must or must not be conveyed by the speakers. (Jakobson 1959:142)

• By and large, constraints become stricter and more arbitrary as one moves down the hierarchy of complexity levels. At the text level, the combination of clauses obeys principles of semantic cohesion, not grammatical constraints. At the clause level, main constituent order may be changed by operations of topicalization and clefting. At the level of the syntagma, the language of b allows no freedom of word order, but other languages including Latin would allow it. Finally at the morphological level, the speaker has no choice but to obey the morphological rules. These, however, are completely arbitrary. There is nothing inherent in the message which would make one expect a person agreement suffix on a verb or an adverbial suffix on an adjective.
• The constraints operate not only on the syntagmatic, but also on the paradigmatic axis. At the highest complexity levels, the speaker may choose expressions that fit his thought and communicative intention. At the text level, the pronominalization illustrated in b is optional. At the clause level, the speaker may choose to say ill instead of sick or hospital instead of physician. At the lowest level, the language system makes the choice for him: the form of the copula must be is, the form of the personal pronoun must be she, the conjugation suffix on the verb need must be -s, the adverbializer appearing on urgent must be -ly. In general, to the extent that allomorphy is not conditioned phonetically, it displays arbitrariness at the lowest level.

In the last instance, a rule of grammar can be arbitrary to the point of producing a counter-iconic configuration. One such example is provided by the 3rd person singular suffix in b, which runs counter to well-motivated principles of markedness theory. Another is the definite article introducing the relative pronoun el que 'who/which' in Spanish, which does not relate to any determination in semantic terms, much less a definite determination ().

There is, thus, a certain correlation between decreasing complexity level and decreasing motivation – thus, increasing arbitrariness. The lowest complexity level is the level of the single morpheme or root; and this is where arbitrariness prevails. The freedom of choice for the speaker decreases with the complexity level. Where the speaker has a choice, his choice will be motivated by the structure of his message. If the speaker cannot choose what to say, then the structure of what he must say, anyway, does not need to be motivated. Other considerations including shortness and increase of redundancy then prevail.