Linguistic typology is a subfield of linguistics that studies and classifies languages according to their structural features. Its aim is to describe and explain the structural diversity of the world's languages. It includes three subdisciplines: qualitative typology, which deals with the issue of comparing languages and within-language variance, quantitative typology, which deals with the distribution of structural patterns in the world’s languages, and theoretical typology, which explains these distributions.
These are usually abbreviated SVO and so forth, and may be called "typologies" of the languages to which they apply.
Some languages split verbs into an auxiliary and an infinitive or participle, and put the subject and/or object between them. For instance, German ("Im Wald habe ich einen Fuchs gesehen" - *"In-the woods have I a fox seen"), Dutch ("Hans vermoedde dat Jan Piet Marie zag leren zwemmen" - *"Hans suspected that Jan Piet Marie saw teach swim") and Welsh ("Mae'r gwirio sillafu wedi'i gwblhau" - *"Is the checking spelling after its to complete"). In this case, typology is based on the non-analytic tenses (i.e. those sentences in which the verb is not split) or the position of the auxiliary. German is thus SVO/VSO (without "im Wald" the agent would go first) in main clauses and Welsh is VAP (and P would go after the infinitive).
Both German and Dutch are often classified as V2 languages, as the verb invariantly occurs as the second element of a full clause.
Some languages allow varying degrees of freedom in their constituent order that pose a problem for their classification. To define a basic constituent order type in this case, one generally looks at frequency of different types in declarative affirmative main clauses in pragmatically neutral contexts, preferably with only old referents. Thus, for instance, Russian is widely considered an SVO language, as this is the most frequent constituent order under such conditions—all sorts of variations are possible, though, and occur in texts. In many inflected languages such as Russian, Latin, and Greek, departures from the default word orders are permissible, but usually imply a shift in focus, an emphasis on the final element, or some special context. In the poetry of these languages, the word order may also be freely shifted to meet metrical demands. Additionally, freedom of word order may vary within the same language—for example, formal, literary, or archaizing varieties may have different, stricter, or more lenient constituent-order strictures than an informal spoken variety of the same language.
On the other hand, when there is no clear preference under the described conditions, the language is considered to have "flexible constituent order" (a type unto itself).
An additional problem is that in languages without living speech communities, such as Latin, Hellenic Greek, and Old Church Slavonic, linguists have only written evidence, perhaps written in a poetic, formalizing, or archaic style that mischaracterizes the actual daily use of the language. The daily spoken language of a Sophocles or a Cicero might have exhibited much different or much more regular syntax than their written legacy indicates.
Many languages show mixed accusative and ergative behaviour (e.g. ergative morphology marking the verb arguments, on top of an accusative syntax). Other languages (called "active languages") have two types of intransitive verbs—some of them ("active verbs") join the subject in the same case as the agent of a transitive verb, and the rest ("stative verbs") join the subject in the same case as the patient. Yet other languages behave ergatively only in some contexts (this is called split ergativity, and is usually based on the grammatical person of the arguments or in the tense/aspect of the verb). For example, only some verbs in Georgian behave this way, and, as a rule, only while the tense called aorist is used.