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 relative clauses in which the nominal head functions as the subject of the relative clause. Relative clauses of this type, known as subject-headed relative clauses, can be found in every language to date. For this reason, the dominant relativizing strategy of subject-headed relative clauses is termed the standard strategy. If no strategy is dominant, all existing strategies are considered the standard.
RelPro: Subject-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. The pronoun is marked as the subject of the relative clause, while the nominal head is marked for the role it plays in the main clause.
CorrelN: Subject-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: Subject-headed relative clauses include both the nominal head in the relative clause, marked as the subject, and a coreferential (personal or demonstrative) pronoun in the main clause, marked for the role it plays in the clause.
RCS: Subject-headed relative clauses include the nominal head in the relative clause, marked as the subject; at the same time, no coreferential noun or pronoun appears in the main clause, nor is the role of the ellipted nominal head in this clause marked.
RCnonS: While the nominal head of a subject-headed relative clause can be understood to be the subject of the clause, this role is not marked grammatically; instead, the noun is only marked for its syntactic role in the main clause. 
Paratakt: Subject-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.
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.
 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 subject of a relative clause is not necessarily the subject of the main clause. Consider, for example, the following sentence: I met the person who will be my neighbor. Although who is the subject of the relative clause, its nominal head, person, is the object of the main clause.
 The dominant strategy, for the purposes of this parameter, is that which occurs with greater frequency.
 Languages of this type display the following structure: *I don’t like the dog that it barks. For a language to display this type, this structure must be considered fully grammatical. (In English, for example, it does not constitute a well-formed relative clause.)
 Languages of this type display the following structure: *Which dog barks that dog /animal doesn’t bite.
 Languages of this type display the following structure: *Which dog barks,it doesn’t bite or *Which dog barks, I don’t like it.
 This type of relative clause can be checked in sentences in which the nominal head would not be the subject of the main clause. English relative clauses of this type include the following: I ate what was available and I want to meet whoever sent the letter.
 This type of relative clause can only be checked in sentences in which the nominal head is not the subject of the main clause. Languages of this type display the following structure: *I am afraid of the dog (of that) barks. This type also includes participial strategies, such as *I give flowers to the smiling at me girl.
 Type *I don’t like the dog, the dog barks.