Editing X-bar theory (section)

=== Basic principles ===
The "X" in the X-bar theory is equivalent to a [[variable (mathematics)|variable]] in mathematics: It can be substituted by [[syntactic categories]] such as [[noun|N]], [[verb|V]], [[adjective|A]], and [[preposition|P]]. These categories are [[lexemes]] and not [[phrases]]: The "X-bar" is a grammatical unit larger than X, thus than a lexeme, and the X-double-bar (=XP) outsizes the X(-single)-bar. X-double-bar categories are equal to [[phrasal category|phrasal categories]] such as [[noun phrase|NP]], [[verb phrase|VP]], [[adjective phrase|AP]], and [[prepositional phrase|PP]].<ref name="J77a" />

The X-bar theory assumes that all [[phrasal categories]] have the structure in Figure 1.<ref name="J77a" /><ref name="C81">{{Cite book|last=Chomsky|first=Noam|authorlink= |title=Lectures on Government and Binding|publisher=MIT Press|location=Cambridge, MA|language= |year=1981|page= |id= |isbn= |quote= }}</ref> This structure is called the '''X-bar schema'''.
[[File:X-bar_schema_(basic).png|thumb|none|300px|Figure 1]]
As in Figure 1, the phrasal category XP is notated by an X with a double overbar.{{refn|group="FN"|Jackendoff (1977a)<ref name="J77a" /> assumes bar-levels up to X-triple-bar.}} For typewriting reasons, the bar symbol is often substituted by the prime ('), as in '''X''''.

{{Anchors|Headedness principle|Binarity principle}}The X-bar theory embodies two central principles.
* '''Headedness principle''': Every phrase has a head.<ref name="radford">{{Cite book|last=Radford|first=Andrew|authorlink= |translator= |year=2016|title=Analysing English Sentences: Second Edition|publisher=Cambridge University Press|location=Cambridge|pages=114–115|id= |isbn= |quote= }}</ref>
* '''Binarity principle''': Every [[node (linguistics)|node]] branches into two different nodes.<ref name="radford" />
The headedness principle resolves the issues 1 and 3 above simultaneously. The binarity principle is important to '''projection''' and ambiguity, which will be explained below.

{{Anchors|Specifier|Head|Complement|Adjunct}}The X-bar schema consists of a head and its circumstantial components, in accordance with the headedness principle.<ref name="J74" /><ref name="J77a" /><ref name="J77b" /><ref name="C81" /> The relevant components are as follows:
* '''Specifier''': '''<nowiki>[</nowiki>obligatory<nowiki>]</nowiki>''' The node that is in a sister relation with an X' node.<ref>{{Cite book|editor-last1=Araki|editor-first1=Kazuo|authorlink= |translator= |year=1999|title=Eigogaku Yogo Jiten (A Dictionary of Technical Terms of English Linguistics)|publisher=Sanseido|location=Tokyo|page=587|id= |isbn= |quote= }}</ref> This is a term that refers to the syntactic position itself.
* '''[[head (linguistics)|Head]]''': '''<nowiki>[</nowiki>obligatory<nowiki>]</nowiki>''' The core of a phrase, into which a [[lexeme]] fits. The head determines the form and characteristics of the phrase as a whole.<ref><!-- name="dict"-->{{Cite book|editor-last1=Araki|editor-first1=Kazuo|authorlink= |translator= |year=1999|title=Eigogaku Yogo Jiten (A Dictionary of Technical Terms of English Linguistics)|publisher=Sanseido|location=Tokyo|page=249|id= |isbn= |quote= }}</ref>
* '''[[complement (linguistics)|Complement]]''': '''<nowiki>[</nowiki>obligatory<nowiki>]</nowiki>''' An [[argument (linguistics)|argument]] required by the head.
* '''[[adjunct (grammar)|Adjunct]]''': '''<nowiki>[</nowiki>optional<nowiki>]</nowiki>''' A modifier for the phrase constituted by the head.
The specifier, head, and complement are obligatory; hence, a phrasal category XP must contain one specifier, one head, and one complement. On the other hand, the adjunct is optional; hence, a phrasal category contains zero or more adjuncts. Accordingly, when a phrasal category XP does not have an adjunct, it forms the structure in Figure 2.
[[File:X-bar_schema_(wo adjunct).png|thumb|none|300px|Figure 2]]
For example, the NP ''linguistics'' in the sentence ''John studies linguistics'' has the structure in Figure 3.
[[File:The_X-bar_structure_of_the_NP_linguistics.png|thumb|none|250px|Figure 3]]
It is important that even if there are no candidates that can fit into the specifier and complement positions, these positions are syntactically present, and thus they are merely empty and unoccupied. (This is a natural consequence of the binarity principle.) This means that all phrasal categories have fundamentally uniform structures under the X-bar schema, which makes it unnecessary to assume that different phrases have different structures, unlike when one adopts the PSR.<ref name="C81" /> (This resolves the second issue above.) In the meantime, one needs to be wary of when such empty positions are representationally omitted as in Figure 4.
[[File:The_X-bar_structure_of_the_NP_linguistics_(simplified).png|thumb|none|130px|Figure 4]]
In illustrating syntactic structures this way, at least one X'-level node is present in any circumstance because the complement is obligatory.<ref name="chom-dict-xbar" /><ref name="C86a">{{Cite book|last=Chomsky|first=Noam|authorlink= |translator= |year=1986a|title=Barriers|publisher=MIT Press|location=Cambridge, MA|page=|id= |isbn= |quote= }}</ref>

{{Anchors|Projection|Zero-level projection|Intermediate projection|Maximal projection}}Next, the X<nowiki>''</nowiki> and X' inherit the characteristics of the head X. This trait inheritance is referred to as '''''projection'''''.<ref>{{Cite book|editor-last1=Araki|editor-first1=Kazuo| author-link= |year=1999|title=Eigogaku Yogo Jiten (A Dictionary of Technical Terms of English Linguistics)|publisher=Sanseido|location=Tokyo|page=489|id= |isbn= |quote= }}</ref>
[[File:X-bar_schema_(projection)_en.png|thumb|none|400px|Figure 5]]
Figure 5 suggests that syntactic structures are derived in a bottom-up fashion under the X-bar theory. More specifically, the structures are derived via the following processes.
# A lexeme is fitted into the head. Heads are sometimes called '''''zero-level projections''''' because they are X-zero-bar-level categories, notated as '''X<sup>0</sup>'''.<ref>{{Cite web |url=http://primus.arts.u-szeged.hu/bese/Glossary/gloss_zero_level_projection.htm|title=Basic English Syntax with Exercises|accessdate=2021-10-22}}</ref>
# The head and the complement are combined to form an X-single-bar (<span style="text-decoration:overline">X</span>, X') node, which constitutes a semi-phrasal category (a syntactic category not as big as a phrase). This category is called '''''intermediate projection'''''.<ref name="chom-dict-xbar" />
# (An adjunct, if there is any, combines with an X' to form another X'. If there is more than one adjunct, this process is repeated.)
# An intermediate projection combines with the specifier, forming a complete phrasal category XP (X-double-bar). This category is called '''''maximal projection'''''.<ref name="chom-dict-xbar" />

It is important that all the processes except for the third are obligatory. This means that one phrasal category necessarily includes X<sup>0</sup>, <span style="text-decoration:overline">X</span>, and XP (=X<nowiki>''</nowiki>). Moreover, nodes bigger than X<sup>0</sup> (thus, <span style="text-decoration:overline">X</span> and XP nodes) are called '''[[constituent (linguistics)|constituents]]'''.<ref>{{Cite book|author=Haraguchi|display-authors=et al.|authorlink= |translator= |title=Zouhoban Chomsky Riron Jiten|publisher=Kenkyusha|location=Tokyo|year=2016|pages=125–126|id= |isbn= |quote= |edition= Kenkyusha's Dictionary of Theoretical Linguistics Enlarged}}</ref>