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.
This parameter considers not only the various types of relativizers (the syntactic and morphological strategies through which languages mark relative clauses), but also the syntactic position in which relativizers appear with respect to the relative clause.
NoRzer: Relative clauses are not marked by a relativizer.
RzerPro: Relative clauses are marked by the use of a relative pronoun; the word order of relative pronouns within the relative clause is not obligatorily clause-initial or clause-final.
InitRzerPro: Relative clauses are marked by the use of an obligatory clause-initial relative pronoun.
FinalRzerPro: Relative clauses are marked by the use of an obligatory clause-final relative pronoun.
RzerComp: Relative clauses are marked by the use of a relative complementizer; the word order of relative complementizers within the relative clause is not obligatorily clause-initial or clause-final.
InitRzerComp: Relative clauses are marked by the use of an obligatory clause-initial relative complementizer.
FinalRzerComp: Relative clauses are marked by the use of an obligatory clause-final relative complementizer.
RzerVerb: Relative clauses are marked on the internal structure of the verb (mood, special verb form, selection of verb, etc.).
RzerSuff: Relative clauses are marked by the use of a relativizing suffix that attaches directly or indirectly to the root of the verb stem.
RzerPref: Relative clauses are exclusively marked by the use of a relativizing prefix that attaches directly or indirectly to the root of the verb stem.
When a language displays more than one strategy, two values can be listed. If one strategy is dominant, a slash (/) can separate the values, with the dominant value appearing first; if neither is dominant, they are listed with an ampersand (&) separating them. The use of parentheses indicates that use of the strategy is not obligatory, while the use of a plus sign (+) instead of an ampersand indicates that the two strategies must be used simultaneously. 
 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.
 Languages that display the value Paratakt, CorrelN, or CorrelPro for the parameter Subject-headed relative clauses fall into this type. If this value applies, the details should be specified in the commentary.
 Relative pronouns do not belong to the category nouns, and their use is limited to marking subordination in relative clauses.
 Clause-initial status still applies if the only word to precede the pronoun is a preposition.
 Clause-final status still applies if the only word to follow the pronoun is a postposition.
 Relative complementizers differ from relative pronouns in that they can appear in other subordinate clauses, such as object clauses.
 This type includes the use of a derivational suffix to form a participle.
 This type includes the use of a derivational prefix to form a participle.
 For example, a langauge that simultaneously employs clause-initial relative pronouns and obligatory use of the subjunctive would be described with the parameter value InitRzerPro+RzerVerb.