In linguistics, word order typology refers to the study of the order of the syntactic constituents of a language, and how different languages can employ different orders. Correlations between orders found in different syntactic subdomains are also of interest. The primary word orders that are of interest are the constituent order of a clause—the relative order of subject, object, and verb; the order of modifiers (adjectives, numerals, demonstratives, possessives, and adjuncts) in a noun phrase; and the order of adverbials.
Some languages have relatively restrictive word orders, often relying on the order of constituents to convey important grammatical information. Others, often those that convey grammatical information through inflection, allow more flexibility which can be used to encode pragmatic information such as topicalisation or focus. Most languages however have some preferred word order which is used most frequently.
For most nominative–accusative languages which have a major word class of nouns and clauses which include subject and object, constituent word order is commonly defined in terms of the finite verb (V) and its arguments, the subject (S) and object (O).   
There are six theoretically possible basic word orders for the transitive sentence: subject–verb–object (SVO), subject–object–verb (SOV), verb–subject–object (VSO), verb–object–subject (VOS), object–subject–verb (OSV) and object–verb–subject (OVS). The overwhelming majority of the world's languages are either SVO or SOV, with a much smaller but still significant portion using VSO word order. The remaining three arrangements are exceptionally rare, with VOS being slightly more common than OSV, and OVS being significantly more rare than the two preceding orders.
It is not always easy to find the basic word order of S, O and V. First, not all languages make use of the categories of subject and object. In others, the subject and object may not form a clause with the verb. If subject and object can be identified within a clause, the problem can arise that different orders prevail in different contexts. For instance, French has SVO for nouns, but SOV when the object is a pronoun and VSO for questions; German has verb-medial order in main clauses, but verb-final order in subordinate clauses. In other languages the word order of transitive and intransitive clauses may not correspond. In still others, the rules for ordering S, O, and V may exist, but be secondary to (and often overruled by) more fundamental ordering rules—e.g., for considerations such as topic–comment. To have a valid base for comparison, the basic word order is defined as:
While the first two of these requirements are relatively easy to respect, the latter two are more difficult. In spoken language, there are hardly ever two full nouns in a clause; the norm is for the clause to have at most one noun, the other arguments being pronouns. In written language, this is somewhat different, but that is of no help when investigating oral languages. Finally, the notion of "pragmatically neutral" is difficult to test. While the English sentence "The king, they killed." has a heavy emphasis on king, in other languages, that order (OSV) might not carry a significantly higher emphasis than another order.
If all the requirements above are met, it still sometimes turns out that languages do not seem to prefer any particular word order. The last resort is text counts, but even then, some languages must be analyzed as having two (or even more) word orders.
These are all possible word orders for the subject, verb, and object in the order of most common to rarest (the examples use "I" as the subject, "see" as the verb, and "him" as the object):
Sometimes patterns are more complex:German, Dutch and Frisian have SOV in subordinates, but V2 word order in main clauses, SVO word order being the most common. Using the guidelines above, the unmarked word order is then SVO.
Others, such as Latin, Romanian and Finnish, have no strict word order; rather, the sentence structure is highly flexible and reflects the pragmatics of the utterance. Nonetheless, there is often a preferred order; in Latin, SOV is the most frequent outside of poetry, and in Finnish SVO is both the most frequent and obligatory when case marking fails to disambiguate argument roles. Just as languages may have different word orders in different contexts, so may they have both fixed and free word orders. For example, Russian has a relatively fixed SVO word order in transitive clauses, but a much freer SV / VS order in intransitive clauses. Cases like this can be addressed by encoding transitive and intransitive clauses separately, with the symbol 'S' being restricted to the argument of an intransitive clause, and 'A' for the actor/agent of a transitive clause. ('O' for object may be replaced with 'P' for 'patient' as well.) Thus Russian is fixed AVO but flexible SV/VS. Such an approach allows the description of word order to be more easily extended to languages which do not meet the criteria in the preceding section. For example, the Mayan languages have been described with the rather uncommon VOS word order. However, they are ergative–absolutive languages, and the more specific word order is intransitive VS, transitive VOA, where S and O arguments both trigger the same type of agreement on the verb. Indeed, many languages claimed to have a VOS word order turn out to be ergative like Mayan.
A fixed or prototypical word order is one out of many ways to ease the processing of sentence semantics and reducing ambiguity. One method of making the speech stream less open to ambiguity (complete removal of ambiguity is probably impossible) is a fixed order of arguments and other sentence constituents. This works because speech is inherently linear. Another method is to label the constituents in some way, for example with case marking, agreement, or another marker. Fixed word order reduces expressiveness but added marking increases information load in the speech stream, and for these reasons strict word order seldom occurs together with strict morphological marking, one counter-example being Persian.
Observing discourse patterns, it is found that previously given information (topic) tends to precede new information (comment). Furthermore, acting participants (especially humans) are more likely to be talked about (to be topic) than things simply undergoing actions (like oranges being eaten). If acting participants are often topical, and topic tends to be expressed early in the sentence, this entails that acting participants have a tendency to be expressed early in the sentence. This tendency can then grammaticalize to a privileged position in the sentence, the subject.
The mentioned functions of word order can be seen to affect the frequencies of the various word order patterns: An overwhelming majority of languages have an order in which S precedes O and V. Whether V precedes O or O precedes V however, has been shown to be a very telling difference with wide consequences on phrasal word orders.
Knowledge of word order on the other hand can be applied to identify the thematic relations of the NPs in a clause of an unfamiliar language. If we can identify the verb in a clause, and we know that the language is strict accusative SVO, then we know that Grob smock Blug probably means that Grob is the smocker and Blug the entity smocked. However, since very strict word order is rare in practice, such applications of word order studies are rarely effective.
The order of constituents in a phrase can vary as much as the order of constituents in a clause. Normally, the noun phrase and the adpositional phrase are investigated. Within the noun phrase, one investigates whether the following modifiers occur before or after the head noun.
Within the adpositional clause, one investigates whether the languages makes use of prepositions (in London), postpositions (London in), or both (normally with different adpositions at both sides).
There are several common correlations between sentence-level word order and phrase-level constituent order. For example, SOV languages generally put modifiers before heads and use postpositions. VSO languages tend to place modifiers after their heads, and use prepositions. For SVO languages, either order is common.
For example, French (SVO) uses prepositions (dans la voiture, à gauche), and places adjectives after (une voiture spacieuse). However, a small class of adjectives generally go before their heads (une grande voiture). On the other hand, in English (also SVO) adjectives almost always go before nouns (a big car), and adverbs can go either way, but initially is more common (greatly improved). (English has a very small number of adjectives that go after their heads, such as "extraordinaire", which kept its position when it was borrowed from French.)
Some languages do not have a fixed word order. In these languages there is often a significant amount of morphological marking to disambiguate the roles of the arguments; however there are also languages in which word order is fixed even though the degree of marking would enable free word order, and languages with free word order, such as some varieties of Datooga, which have free word order combined with a lack of morphological distinction between arguments. Typologically there is a trend that highly animate actors are more likely to be topical than low-animate undergoers, this trend would come through even in free-word-order languages giving a statistical bias for SO order (or OS in the case of ergative systems, however ergative systems do not usually extend to the highest levels of animacy, usually giving way to some form of nominative system at least in the pronominal system). Most languages with a high degree of morphological marking have rather flexible word orders such as Latin, Portuguese, Romanian, Serbian, Hungarian, Croatian, Russian (in intransitive clauses), and Finnish. In some of those, a canonical order can still be identified, but in others this is not possible.
In Hungarian, the enclitic -t marks the direct object. For "Kate ate a piece of cake", the possibilities are:
In Portuguese, the clitic pronouns allow many different orders:
In many languages, changes in word order occur due to topicalization or in questions. However, most languages are generally assumed to have a basic word order, called the unmarked word order; other, marked word orders can then be used to emphasize a sentence element, to indicate modality (such as an interrogative modality), or for other purposes.
For example, English is SVO (subject-verb-object), as in "I don't know that", but OSV is also possible: "That I don't know." This process is called topic-fronting (or topicalization) and is common. In English, OSV is a marked word order because it emphasises the object, and is often accompanied by a change in intonation.
An example of OSV being used for emphasis:
A: I can't see Alice. (SVO)
B: What about Bill?
A: Bill I can see. (OSV, rather than I can see Bill, SVO)
Non-standard word orders are also found in poetry in English, particularly archaic or romantic terms – as the wedding phrase “With this ring, I thee wed” (SOV) or “Thee I love” (OSV) – as well as in many other languages.
Differences in word order complicate translation and language education – in addition to changing the individual words, the order must also be changed. This can be simplified by first translating the individual words, then reordering the sentence, as in interlinear gloss, or by reordering the words prior to translation, as in English-Ordered Japanese. See reordered languages for further examples.