An essential (or restrictive) relative clause is a referring expression that restricts the set of possible entities denoted by the head noun to a particular entity or set of entities that meet the criteria specified in this clause. A relative clause fulfills both of the following criteria:
- Taken as a whole, the clause functions as an attribute of the noun (N), answering the question What is the N like? or Which N?
- The structure contains a finite or nonfinite verb form distinct from the predicate of the main clause.
Together, the relative clause and the nominal head it modifies constitute the relative construction. Languages use a variety of relativizing strategies to mark relative clauses. Strategies worth consideration from a typological perspective are expressed morphosyntactically. The dominant relativizing strategy of a language is the type employed by its subject-headed relative clauses.
This parameter considers relative clauses in which the nominal head fulfills an oblique role in the sentence.  Relative clauses of this type, known as oblique-headed relative clauses , may be expressed through a variety of morphological and syntactic strategies. Oblique refers here to the following the non-subject, non-object roles: (1) indirect object, (2) adverbial describing the circumstances under which an event occurred (such as the comitative and instrumental), (3) possessor (genitive), and (4) standard of comparison.
NoOblRel: The language does not have oblique-headed relative clauses.
ElseOblRel: Oblique-headed relative clauses are expressed only by means other than the standard relativizing strategy.
RCnonObl: While the nominal head of an oblique-headed relative clause can be understood to fulfill an oblique role in the clause, this role is not marked grammatically; instead, the noun is only marked for its syntactic role in the main clause.
RelPro: Oblique-headed relative clauses include both the nominal head of the relative clause in the main clause and an obligatory coreferential pronoun in the relative clause, generally marked for its oblique role.
CorrelN: Oblique-headed relative clauses include both the nominal head in the relative clause and either the same noun or a more generic term in the main clause.
CorrelPro: Oblique-headed relative clauses include both the nominal head in the relative clause, marked for its oblique role, and a coreferential (personal or demonstrative) pronoun in the main clause, marked for the role it plays in the clause.
InternN: The nominal head appears only in the relative clause; no coreferential noun or pronouns appears in the main clause.
Paratact: Oblique-headed relative clauses are formed by the juxtaposition of two main clauses, without the use of subordination or coordination. In this type of relative clause, known as a paratactic relative clause, neither clause contains any morphological features that refer to the other clause.
Pro: A pronoun obligatorily appears in the relative clause to mark the syntactic role of the nominal head within the clause.
When a language displays more than one type, multiple values can be listed. If one type is dominant, a slash (/) can separate the two values, with the dominant value appearing first; if neither is dominant, they are listed with an ampersand (&) separating the two. The use of parentheses indicates that the strategy is not obligatory. The values NoOblRel and ElseOblRel may appear alongside another value, listed with a plus sign (+), if the standard strategy only applies to particular oblique subtypes.  Such cases should be described in the commentary.
 If the relative clause is removed from the sentence, the main clause should be able to stand alone as a grammatical clause.
 For example, in the construction the dog that barks, the nominal head is the dog, while the relative clause is that barks. In the participle-based example the barking dog, the nominal head is dog, while the relative clause is barking.
 The dominant strategy, for the purposes of this parameter, is that which occurs with greater frequency.
 Subject-headed relative clauses can be found in every language to date.
 The oblique of a relative clause is not necessarily oblique in the main clause. Consider, for example, the following sentence: This is the key with which you can open the lock. The relative pronoun which is the object of the preposition, expressing an instrumental adverbial, while the nominal head key is the subject or predicate of the main clause.
 For example, a language might rely on a construction, in which the nominal head appears as the subject instead of in an oblique role, as in I know the person who was your walking partner yesterday, instead of I know the person you walked with yesterday.
 For example, some oblique subtypes may use the standard strategy while the rest cannot form oblique-headed relative clauses, as in RelPro+NoOblRel. In another possible variation, some subtypes rely on the standard strategy while others use non-standard strategies (RelPro+ElseOblRel).