Relative clauses are defined here as clauses that fulfill 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.
A noun modified by a relative clause is considered the head of the NP. As the head, this noun may appear either within or outside the relative clause.
This parameter describes the syntactic relationship of the relative clause to its head.
ExtRel: The head appears outside the relative clause. This is called an external relative clause.
IntRel: The head appears within the relative clause. This is called an internal relative clause.
CorrRel: The head appears both within the relative clause and as an anaphoric pronoun alongside the predicate of the main clause. This is called a correlative relative clause.
AdjnRel: The head obligatorily appears in the main clause, and the relative clause contains no anaphoric referent to the head, neither noun nor pronoun. This is called an adjoined relative clause.
DblhdRel: The head obligatorily appears in both the main clause and the relative clause. (Alternatively, the main clause may contain a noun with a more general meaning, rather than the exact same noun.) This is called a double-headed relative clause.
When a language displays more than one type, two values can be listed. If one type is dominant, appearing in a greater variety of environments, 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.
 If the relative clause is removed from the sentence, the main clause should be able to stand alone as a grammatical clause. The relative clause as defined here describes a broader category than in traditional usage of the term, as it has been expanded to include phrases containing a nonfinite, rather than finite, verb. For this reason, the underlined sections in the following English examples are equally considered to be relative clauses: The dog that is chasing the cat is barking; The dog chasing the cat is barking loudly.
 Consider the following examples from English: whichever dog barks… and the dog that barks…. The former is an internal relative clause, while the latter is an external relative clause.
 In addition to the entire clause being subordinate, a grammatical word or morpheme may also indicate that the phrase or clause is a relative clause.