The penthouse principle, a term in syntax

Syntax

In linguistics, syntax is the study of the principles and rules for constructing phrases and sentences in natural languages....

 coined by John R. Ross

John R. Ross

John Robert "Haj" Ross is a linguist who played a part in the development of generative semantics along with George Lakoff, James D. McCawley, and Paul Postal...

 in 1973, describes the fact that many syntactic phenomena treat matrix (or main) clauses differently from embedded (or subordinate) clauses:

The penthouse principle: The rules are different if you live in the penthouse.

The penthouse

Penthouse apartment

A penthouse apartment or penthouse is an apartment that is on one of the highest floors of an apartment building. Penthouses are typically differentiated from other apartments by luxury features.-History:...

named in the principle is the top-floor of a highrise apartment building, and is a metaphor for the matrix clause in a multi-clause structure (which, when diagrammed in usual phrase marker notation, contains the highest clause node in the structure). Perhaps the best-known example of a penthouse principle effect is the distribution of subject-auxiliary inversion

Subject-auxiliary inversion

In English, subject-auxiliary inversion occurs when an auxiliary verb precedes a subject. This is an exception to the English word order convention of subjects preceding their corresponding verbs...

in constituent questions in English, which in many (but not all) varieties of English is restricted to matrix clauses:
a. What can Sam do about it?

b. I'll find out what Sam can do about it.

Compare: a. *What Sam can do about it?

b. *I'll find out what can Sam do about it.

Other phenomena falling under the penthouse principle are V2-effects in the Germanic languages and the distribution of declarative markers, imperative morphology, and of various particles in a variety of languages.