Certain syntactic structures are islands; i.e, they are effectively isolated from the rest of the sentence they are in, for the purpose of various kinds of rules, typically those which involve licensing the appearance of sentence elements in unexpected places in the structure, while suppressing them in their "original" (i.e, expected) position.

Viewed in a temporal perspective on a derivation, this produces a metaphor of moving them from one place to another. Hence, these rules are called "Movement" rules (actually, linguists rarely use scare quotes around movement; we know it's just a metaphor. But I retain them here for the uninitiated). Prototypic examples are Wh-Question Formation (where the "Movement" is of the wh- interrogative pronoun to the beginning of the sentence) and Relative Clause Formation (which moves the relative pronoun to the beginning of the clause).

There are basically two types of such "Movement" rules:

1. Rules that operate only inside a single clause or between adjacent clauses, like Goal Advancement, also called Dative Movement or Dative Alternation, which relates the following:
   • Bill brought the book to Mary.
     Bill brought Mary the book.

   • Bill bought the book for Mary.
     Bill bought Mary the book.

   This kind of rule is sometimes called a cyclic rule. Cyclic rules are governed by the predicate in the relevant clause, and often have lexical exceptions. Equi-NP-Deletion and Subject-Raising are other examples of cyclic rules. Islands are not a relevant concept to cyclic rules, because the scope of cyclic rules is already limited.

2. Rules that "operate over variables" - i.e, arbitrarily large chunks of sentence can appear between the expected ("original") site and the actual location of the "moved" element. Such rules are not governed by lexical items, and do not have lexical exceptions.
   These are the rules to which the syntactic concept of "island" is relevant.

   Some examples:

   • Bill wants to try to begin to learn to speak Spanish this year.

     What language_i does Bill want to try
     to begin to learn to speak ____i this year?
     (Wh-Question Formation)

   • Sue thinks you said that I heard that Fran claimed that Bill likes Mary.

     Mary_i is the person_i who_i Sue thinks you said that
     I heard that Fran claimed that Bill likes ____i.
     (Relative Clause Formation)

   The metaphor of "island" comes from the fact that Type 2 "Movement" rules cannot move a constituent grammatically over an island boundary. Application of these rules over island boundaries produces ungrammaticality.

   Islands are described in terms of constraints on Type 2 "Movement" rules, each constraint describing an island category. Below are a few examples of such island constraints; the complete dissertation can be downloaded.

Coordinate Structures are those joined with a Coordinating Conjunction, like and, or, but, etc. An element from one conjunct cannot be moved out of that structure:

• Bill cooked supper.         What did Bill cook?
• Bill washed the dishes.    What did Bill wash?
• Bill cooked supper and washed the dishes.
• *What_i did Bill cook ____i and wash the dishes?
• *What_i did Bill cook supper and wash ____i?

I.e, coordinate structures are islands.
There are some interesting exceptions to this, such as

• Bill washed the dishes_i and Fred dried them_i.
• What_i did Bill wash ____i and Fred dry ____i?
• Bill went to the store and bought some fish.
• What_i did Bill go to the store and buy ____i?

A Complex NP is one with a noun head and a modifying clause.
There are two types:

1. Relative Clauses, which always contain a noun coreferential to the head noun:

   • [ the dog_i [(which_i) Mary saw ___i]  ]
      NP         S                    S NP

2. NP Complements, which are only possible where the head noun is a picture noun, i.e, a noun denoting a symbolic representation like picture, story, rumor, etc; the complement denotes the content:

   • [ the report_i [that Quayle sleeps with a Teddy Bear]  ]
      NP           S_i                                  S_i NP

• Bill has brothers [living in 3 cities].
  (Participial Clause; not an island)
      How many cities_i does Bill have brothers [living in ____i]?
• Bill has brothers_j [who_j live in 3 cities].
  (Relative Clause: an island)
      *How many cities_i does Bill have brothers_j [who live in ____i]?
• Frank believes [Quayle sleeps with a teddy bear].
  (Finite Complement; not an island)
      What_i does Frank believe [Quayle sleeps with ____i]?
• Frank believes [the report [that Quayle sleeps with a teddy bear]].
  ([NP[Complement]]; an island)
      *What_i does Frank believe [the report [that Quayle sleeps with ____i]]?

Violations of Ross Constraints are very ungrammatical. Most people never encounter them. We appear to formulate our discourse to avoid them. Occasionally, we get in a bind and see one looming at the end of the clause, and have to do something quick. What we do is often illuminating about the relative importance of syntactic rules.

For instance, consider the following:

• ?That's the book_i [that Bill married the woman_j [who_j illustrated it_i]].
• *That's the book_i [that Bill married the woman_j [who_j illustrated ___i]].

Neither sentence is terrifically grammatical, but the first seems more appropriate (and common as a type) than the second, though the last word in the first sentence still feels strange. The ordinary rule of relative clause formation operating on the last clause should result in its deletion at the end of the clause (and thus the sentence). However, it appears inside another relative, an island, and is thus safe from such "Movement" by the Complex NP Constraint.

Sentences like the first one are generated when, at the last minute, the speaker realizes what is going to result, and cancels the deletion, substituting an alternative relative-formation rule (called a Resumptive Pronoun in the trade), which merely pronominalizes the coreferential NP, instead of deleting it in the object position.

This is not the way English forms its relative clauses (though other languages use it frequently, e.g, Hebrew), and the sentence is thus ungrammatical. But this turns out to be a venial syntactic sin by comparison with a violation of a Ross constraint, which typically produces extreme ungrammaticality.