In linguistics, coordination is a frequently occurring complex syntactic structure that links together two or more elements, known as conjuncts or conjoins. The presence of coordination is often signaled by the appearance of a coordinator (coordinating conjunction), e.g. and, or, but (in English). The totality of coordinator(s) and conjuncts forming an instance of coordination is called a coordinate structure. The unique properties of coordinate structures have motivated theoretical syntax to draw a broad distinction between coordination and subordination. Coordination is one of the most studied fields in theoretical syntax, but despite decades of intensive examination, theoretical accounts differ significantly and there is no consensus about the best analysis.
Basic examples 
Coordination is a very flexible mechanism of syntax. Any given lexical or phrasal category can be coordinated. The examples throughout this article employ the convention whereby the conjuncts of coordinate structures are marked using square brackets and bold script. The coordinate structure each time includes all the material that follows the left-most square bracket and precedes the right-most square bracket. The coordinator appears in normal script between the conjuncts.
- [Sarah] and [Xolani] went to town - N + N
- [The chicken] and [the rice] go well together. - NP + NP
- The president will [understand] and [agree]. - V + V
- The president will [understand the criticism] and [take action] - VP + VP
- Insects were [in], [on], and [under] the bed. - P + P + P
- [After the announcement] but [before the game], there was a celebration. - PP + PP
- Susan works [slowly] and [carefully]. - Adv + Adv
- Susan works [too slowly] and [overly carefully]. - AdvP + AdvP
- We appreciated [that the president understood the criticism] and [that he took action]. - Clause + Clause
Data of this sort could easily be expanded to include every lexical and phrasal category. An important aspect of these data is that the conjuncts each time are indisputably constituents. In other words, the material enclosed in brackets would qualify as a constituent in both phrase structure grammars and dependency grammars.
A coordinator (= coordinating conjunction) often appears between the conjuncts, usually at least between the penultimate (second to last) and ultimate (last) conjunct of the coordinate structure. The words and and or are by far the most frequently occurring coordinators in English. Other coordinators occur less often and have unique properties, e.g. but, as well as, then, etc. The coordinator usually serves to link the conjuncts and indicate the presence of a coordinate structure. Depending on the number of coordinators used, coordinate structures can be classified as syndetic, asyndetic, or polysyndetic.
Unique behavior 
Most coordinate structures are like those just produced above; the coordinated strings are alike in syntactic category. There are a number of unique traits of coordination, however, that demonstrate that what can be coordinated is not limited to the standard syntactic categories. Each of the following subsections briefly draws attention to a perhaps unexpected aspect of coordination. These aspects are less than fully understood, despite the attention that coordination has received in theoretical syntax.
Nested coordinate structures 
One coordinate structure can easily be nested inside another. Ambiguity is sometimes the result, e.g.
- Fred and Bill and Sam came.
- a. [Fred] and [Bill] and [Sam] came.
- b. [Fred] and [[Bill] and [Sam]] came.
- c. [[Fred] and [Bill]] and [Sam] came.
The brackets indicate the three possible readings for the sentence. The (b)- and (c)-readings show one coordinate structure being embedded inside another. Which of the three readings is understood depends on intonation and context. The (b)-reading could be preferred in a situation where Bill and Sam arrived together, but Fred arrived separately. Similarly, the (c)-reading could be preferred in a situation where Fred and Bill arrived together, but Sam arrived separately. That the indicated groupings are indeed possible becomes evident when or is employed:
- b'. [Fred] or [[Bill] and [Sam]] came.
- c'. [[Fred] and [Bill]] or [Sam] came.
A theory of coordination needs to be in a position to address nesting of this sort.
Mismatch in syntactic category 
The examples above illustrate that the conjuncts are often alike in syntactic category. There are, though, many instances of coordination where the coordinated strings are NOT alike, e.g.
- Sarah is [a CEO] and [proud of her job]. - NP + AP
- Is Jim [conservative] and [a closet Republican]? - A + NP
- Bill is [in trouble] and [trying to come up with an excuse]. - PP + VP
- Sam works [evenings] and [on weekends]. - Adv + PP
- They are leaving [due to the weather] and [because they want to save money]. - PP + Clause
Data like these have been explored in detail. They illustrate that the theory of coordination should not rely too heavily on syntactic category to explain the fact that in most instances of coordination, the coordinated strings are alike. Syntactic function is more important, that is, the coordinated strings should be alike in syntactic function. In the former three sentences here, the coordinated strings are, as complements of the copula is, predicative expressions, and in the latter two sentences, the coordinated strings are adjuncts that are alike in syntactic function (temporal adjunct + temporal adjunct, causal adjunct + causal adjunct).
Non-constituent conjuncts 
The aspect of coordination that is perhaps most vexing for theories of coordination concerns non-constituent conjuncts. Coordination is, namely, not limited to coordinating just constituents, but rather it is quite capable of coordinating non-constituent strings:
- [When did he] and [why did he] do that?
- [She has] but [he has not] understood the task.
- Susan [asked you] but [forced me] to read the book on syntax.
- [Jill has been promising] but [Fred is actually trying] to solve the problem.
- [The old] and [the new] submarines submerged side-by-side.
- [Before the first] and [after the second] presentation, there will be coffee.
- Fred sent [Uncle Willy chocolates] and [Aunt Samantha ear rings].
- We expect [Connor to laugh] and [Jilian to cry].
While some of these coordinate structures require a non-standard intonation contour, they can all be acceptable. This situation is problematic for theories of syntax because most of the coordinated strings do not qualify as constituents. Hence since the constituent is widely assumed to be the fundamental unit of syntactic analysis, such data seem to require that the theory of coordination admit additional theoretical apparatus. Two examples of the sort of apparatus that has been posited are so-called conjunction reduction and right node raising (RNR). Conjunction reduction is an ellipsis mechanism that takes non-constituent conjuncts to be complete phrases or clauses at some deep level of syntax. These complete phrases or clauses are then reduced down to their surface appearance by the conjunction reduction mechanism. The traditional analysis of the phenomenon of right node raising assumed that in cases of non-constituent conjuncts, a shared string to the right of the conjuncts is raised out of VP in such a manner that the material in the conjuncts ends up as constituents. The plausibility of these mechanisms is NOT widely accepted, but rather one can argue that they are ad hoc attempts to solve a problem that plagues theories that take the constituent to the be fundamental unit of syntactic analysis.
Coordination has been widely employed as a test or for the constituent status of a given string, i.e. as a constituency test. In light of non-constituent conjuncts however, the helpfulness of coordination as a diagnostic for identifying constituents can be doubted.
Gapping or not? 
Gapping (and stripping) is an ellipsis mechanism that seems to occur in coordinate structures only. While gapping itself is widely acknowledged to involve ellipsis, just which instances of coordination do and do not involve gapping is a matter of debate. Most theories of syntax agree that gapping is involved in the following cases. A subscript and a smaller font are used to indicate the "gapped" material:
- [Brent ate the beans], and [Bill ate the rice]. - Gapping
- [You should call me more], and [I should call you more]. - Gapping
- [Mary always orders wine], and [Sally always orders beer]. - Gapping
Accounts of gapping and coordination disagree, however, concerning data like the following:
- a. [They saw him first] and [they saw her second]. - Gapping analysis
- b. They saw [him first] and [her second]. - Non-gapping analysis
- a. [Tanya expects the dog to eat cat food] and [she expects the cat to eat dog food]. - Gapping analysis
- b. Tanya expects [the dog to eat cat food] and [the cat to eat dog food]. - Non-gapping analysis
The gapping analysis shown in the a-sentences is motivated above all by the desire to avoid the non-constituent conjuncts associated with the b-sntences. No consensus has been reached about which analysis is better.
Forward vs. backward sharing 
Coordination is sensitive to the linear order of words, a fact that is evident with differences between forward and backward sharing. There is a limitation on material that precedes the conjuncts of a coordinate structure that does restrict the material that follows it:
- *After Wallace fed [his dog the postman] and [his sheep the milkman] arrived. - Forward sharing fails.
- *The man [who built the rocket has] and [who studied robots designed] a dog. - Forward sharing fails.
- *After [Sue’s presentation , I was sad] and [Fred’s presentation, I was angry]. - Forward sharing fails.
The star * indicates that the sentence is bad. Each of these coordinate structures is disallowed. The underline draws attention to a constituent that mostly precedes the coordinate structure but that the initial conjunct "cuts into". There is apparently a restriction on the constituents that mostly precede a coordinate structure. The same restriction does not limit similar constituents that mostly follow the coordinate structure:
- [She stated the strengths], and [he mentioned the weaknesses] of the explanation. - Backward sharing succeeds.
- [Larry put a flier on], and [Sue slipped one under] the door. - Backward sharing succeeds
- Sally [arrived just before the speaker initiated], and [left right after he finished] his speech. - Backward sharing succeeds
The underline now marks a constituent that mostly follows the coordinate structure. Unlike with the first three examples, the coordinate structure in these three examples CAN cut into the underlined constituent.
In Transformational Grammar, the interaction of coordination and extraction (e.g. wh-fronting) has generated a lot of interest. The Coordinate Structure Constraint is the property of coordinate structures that prevents extraction of a single conjunct or from a single conjunct. Coordinate structures are said to be strong islands for extraction. For example:
- *Who did you see [Fred] and [ ]? - Failed extraction of an entire conjunct
- *Who did you see [ ] and Susan? - Failed extraction of an entire conjunct
- *Which action did the president understand [the criticism] and [take ]? - Failed extraction out of a single conjunct
These attempts at coordination fail because extraction cannot affect just one conjunct of a coordinate structure. If extraction occurs out of both conjuncts in a like fashion, however, the coordinate structure is acceptable. This trait of coordination is referred to as the Across-the-Board Constraint. For example:
- What does [Sarah like ] and [Xolani hate ]? - Across-the-board extraction of What
There are other apparent exceptions the Coordinate Structure Constraint and the Across-the-Board generalization and their integration to existing syntactic theory has been a long-standing disciplinary desideratum.
In pseudo-coordinative constructions, the coordinator, generally and, appears to have a subordinating function. It occurs in many languages and is sometimes known as "hendiadys", and it is often, but not always, used to convey a pejorative or idiomatic connotation. Among the Germanic languages, pseudo-coordination occurs in English, Afrikaans, Norwegian, Danish and Swedish. Pseudo-coordination appears to be absent in Dutch and German. The pseudo-coordinative construction is limited to a few verbs. In English, these verbs are typically go, try, and sit. In other languages, typical pseudo-coordinative verbs and/or hendiadys predicates are egressive verbs (e.g. go) and verbs of body posture (e.g. sit, stand and lie down).
- Why don't you go and jump in the lake
- I will try and jump in the lake
- The pupils sat and read their textbooks
A typical property of pseudo-coordinative constructions is that, unlike ordinary coordination, they appear to violate the Across-the-Board extraction property (see above). In other words, it is possible to extract from one of the conjuncts.
- What did she go and jump in t?
- What did she try and jump in
- Which textbooks did the pupils sit and read.
It has been argued that pseudo-coordination is not a unitary phenomenon. Even in a single language such as English, the predicate try exhibits different pseudo-coordination properties to other predicates and other predicates such as go and sit can instantiate a number of different pseudo-coordinative construction types. On the other hand, it has been argued that at least some different types of pseudo-coordination can be analyzed using ordinary coordination as opposed to stipulating that pseudo-coordinative and is a subordinator; the differences between the various constructions derive from the level of structure that is coordinated e.g. coordination of heads, coordination of VP, etc.
The structure of coordination 
Theoretical accounts of coordination vary in major respects. For instance, constituency- and dependency-based approaches to coordination differ significantly, and derivational and representational systems are also likely to disagree on many aspects of how coordination should be explained, derivational accounts, for instance, being more likely to assume transformational mechanisms to "rectify" non-constituent conjuncts (e.g. conjunction reduction and RNR, as mentioned above).
Even concerning the hierarchical structure of coordinated strings, there is much disagreement. Whether or not coordinate structures should be analyzed in terms of the basic tree conventions employed for subordination is an issue that divides experts. Broadly speaking, there are two options: either a flat or a layered analysis. There are two possibilities for the flat option, both of which are shown here. The a-trees represent the analyses in a constituency-based system, and the b-trees in a dependency-based system:
The first two trees present the traditional exocentric analysis. The coordinate structure is deemed exocentric insofar as neither conjunct can be taken to be the sole head, but rather both conjuncts are deemed heads in a sense. The second two trees, where the coordinator is the head, are similar to the first two insofar as the conjuncts are equi-level sisters. These two flat analyses stand in contrast to the following three layered analyses. The constituency-based a-trees appear again on the left, and the dependency-based b-trees on the right:
The primary aspect of these layered analyses is that an attempt is being made to adapt the analysis of coordinate structures to the analysis of subordinate structures. The conjuncts in each case are NOT sister constituents, but rather the first conjunct is in a more prominent (higher) hierarchical position than the second conjunct. The three analyses differ with respect to the presumed head of the entire structure. The third option in terms of the X-bar schema cannot be rendered in terms of dependency because dependency allows a word to project just a single node. There is no way to capture the hierarchical distinction between specifiers and complements in a dependency-based system (but there is always a linear distinction, since specifiers precede complements).
The flat analysis has the benefit that it captures our intuition that coordinate structures are different from subordinate structures at a basic level. The drawback to the flat analysis, however, is that the theory of syntax must be augmented beyond what is necessary for standard subordinate structures. The layered analysis has the advantage that there is no need to augment the syntax with an additional principle of organization, but it has the disadvantage that it does not sufficiently accommodate our intuition that coordination is fundamentally different from subordination. Most if not all of the nine analyses just presented have been proposed in the massive body of literature on coordination.
See also 
- Concerning the distinction between subordination and coordination, see Payne (2006:309).
- See Williams, E. (1978) concerning the matching conjuncts of coordinate structures.
- See for instance Dik (1968), Sag et al. (1985), Zoerner (1995), Bayer (1996), and Progovac (1998).
- See Osborne (2008).
- Concerning conjunction reduction, see for instance Akmajian and Heny (1980:261f.).
- Concerning RNR, see for instance Hudson (1984:2335f.) and McCawley (1988:56).
- Concerning this debate, see Sag et al. (1985) and Osborne (2006).
- The first two examples are taken from Phillips (2003). All six examples in this section appear in Osborne (2008: 1121).
- See Ross, J. (1967).
- See Ross (1967) and Williams (1978).
- See Carden and Pesetsky (1977), Goldsmith (1985), Lakoff (1986), Zoerner (1995), Culicover and Jackendoff (1997), Progovac (1998).
- See Na and Huck (1992).
- See Wiklund (2005) and De Vos (2005).
- See De Vos (2005) and Lakoff (1986).
- See De Vos (2005).
- Akmajian, A. and F. Heny. 1980. An introduction to the principle of transformational syntax. Cambridge, MA: The MIT Press.
- Bayer, S. 1996. The coordination of unlike categories. Language 72, 579-616.
- Carden, G. and D. Pesetsky 1977. Double-verb constructions, markedness and a fake coordination. In Papers from the 13th regional meeting, Chicago Linguistic Society, Chicago, 82-92. University of Chicago. Reprinted in: Minoru Yasui (ed.), Kaigai Eigogaku-ronso, 1979, Tokyo: Eichosha Company.
- Culicover, P. and R. Jackendoff 1997. Semantic subordination despite syntactic coordination. Linguistic Inquiry 28, 2, 195-217.
- Dik, S. 1968. Coordination: Its implications for a theory of general linguistics. Amsterdam: North Holland Publishing Company.
- Goldsmith, J. 1985. A principled exception of the coordinate structure constraint. In W. Eilfort, P. Kroeber and K. Peterson (eds). CLS 21, Part 1: Papers from the general session at the twenty-first regional meeting, Chicago, 133-143. Chicago Linguistic Society.
- Hudson, R. 1984. Word Grammar. Oxford: Blackwell.
- Lakoff, G. 1986. Frame semantic control of the coordinate structure constraint. In A. Farley, P. Farley, and K-E. McCullough (eds), CLS 22, Part 2: Papers from the parasession on pragmatics and grammatical theory, Chicago, 152-167. Chicago Linguistic Society.
- McCawley, T. 1988. The syntactic phenomena of English, Vol. 1. Chicago: The University of Chicago Press.
- Na, Y. and G. Huck 1992. On extracting from asymmetrical structures. In D. Brentari, G. Larson and L. Macleod (eds), The joy of grammar: A festschrift in honour of James D. McCawley, 251-274. Amsterdam: John Benjamins.
- Osborne, T. 2006. Gapping vs. non-gapping coordination. Linguistische Berichte 207, 307-338.
- Osborne, T. 2008. Major constituents: And two dependency grammar constraints on sharing in coordination. Linguistics 46, 6, 1109-1165.
- Phillips, C. 2003. Linear order and constituency. Linguistic Inquiry 34, 1, 37–90.
- Payne, T. 2006. Exploring language structure: A student's guide. Cambridge, UK: Cambridge University Press.
- Progovac, L. 1998. Structure for coordination (Part 1). GLOT International 3, 7, 3-6.
- Ross, J. 1967. Constraints on variables in syntax. Ph.D. thesis. Massachusetts Institute of Technology.
- Sag, I., Gazdar, T., Wassow, T. and S. Weisler. 1985. Coordination and how to distinguish categories. Natural Language and Linguistic Theory 3, 117-171.
- De Vos, M. 2005. The syntax of pseudo-coordination in English and Afrikaans. Utrecht, the Netherlands: LOT.
- Wiklund, A-L. 2005. The syntax of tenselessness: On copying constructions in Swedish. Ph.D. Thesis. University of Umeaa.
- Williams, E. 1978. Across the board rule application. Linguistic Inquiry 9, 31-43.
- Zoerner, E. 1995. Coordination: The syntax of &P. Ph.D. thesis. University of California, Irvine.
- Postal, P. 1998. Three investigations of extraction. Cambridge MA: MIT Press.
- Schmerling, S. 1975. Asymmetric coordination and rules of conversation. In P. Cole and J. Morgan (Eds). Speech Acts, Volume 3 of Syntax and semantics, pp211–231. New York: Academic Press.