The study of correlating features of language units unevitably leads to valency theory. First the term valency was introduced by the representative of the French school of structural linguistics L. Tenjer who borrowed the term from chemistry. In domestic linguistics this term was introduced by C.D. Katsnelson.
First this term was used only for description of combinative ability of a verb. Then the meaning of the term was widened and now linguists suppose that valency features are typical for a wide range of morphological units.
Tenjer's theory is based on original treatment of a verb. According to his theory a verb is a central unit in the sentence. All the rest members of the sentence including a subject are subordinated to it. But the valency set of a verb included only a subject and objects and these elements were named actants (актанты) i.e. participants of the action. All kinds of adverbial modifiers (sirconstants - сирконстанты) were excluded from the valency set. Coming from semantic analysis Tenjer thought that any action demands definite number of actants which at sentence level can be represented by a subject and an object. According to his opinion all adverbial elements are not conditioned by a verb meaning and that is why they are not members of the verb valency set. Later on this point of view was corrected and some types of adverbial modifiers were also included in a valency set.
This approach helps to make up classification of verbs in correspondence with a number of actants demanded and thus determine the types of phrases they are capable to form.
The further development of the theory brought to some changes in the meaning of the term. Nowadays it is sometimes applied to determine the potential ability of a language unit to combination or only for determination of speech realization of these capabilities.
Facultative and obligatory combining. The idea of valency properties of morphological units is close to the idea of Russian linguists of facultative and obligatory combining.
Obligatory combining is typical of those dependent elements which are demanded by both semantics and a form of language units. Obligatorycombining is typical for the units having "strong government".
The theory of facultative and obligatory combining got its development in the works of V.V.Vinogradov and V.Y.Admoni. Developing the idea V.Y.Admoni includes into the notion not only combining of certain classes of words and morphological forms but also members of the sentence.
Secondary members of a sentence are dedicated to obligatory combining with their key word. Though in a number of syntactic structures there are considerable divergences between semantic demands and a form of separate units on the one hand and their syntactic functioning on the other.
Types of syntactic links in word combinations. Traditionally in syntax there are two main types of links: coordination and subordination. Besides this two member row, another four member row marking predicative, objective, attributive and adverbial relations is considered. That made it possible to introduce into two member row one more type of relations - predicative. L.Elmslev described them in the following way: 1) both elements are independent of each other (what coincides with coordination); 2) the first element depends on the second, but the second doesn't depend on the first (subordination); 3) the first element depends on the second and the second depends on the first (what may be considered as predicative link). L.Elmslev called it interdependence.
Coordination. In modern syntax there are considered symmetrical coordinative groups where components may exchange their places and nonsymmetrical where the places of elements are fixed. It is stated that in binary structures first place is occupied by an element consisting of a fewer number of syllables: "men" and "women", "red" and "green".
At present the universally accepted definition of coordination is the following. Coordinative are considered the groups consisting of relatively independent elements capable of being united with the help of one of coordinative conjunctions.
Theoretically the coordinative row may be prolonged without any limitations, but in speech it seldom includes more than 15 elements.
Subordination is based on unequality of the combining units when one of the elements dominates the others and subordinates them in a form and a place. Dominating unit is called 'nucleus of a phrase' and may be expressed by different parts of speech.
In English subordinative structures are more frequent than coordinative.
Subordinative structures differ in their inner structure and may have the left position of subordinated elements (regressive structures): an old brownstone house. And the right position (progressive structures): a list of names, to put it in the envelope.
There are also subordinative structures with central position of subordinated elements: a folded sheet of paper, laid (больной, нездоровый).
Subordinative structures are limited in their membership but nominative phrase with a noun as nucleus and post positional attribute expressed by prepositional group may have no right border (theoretically): 'the man in the store across the street by the bank under the bridge'..
In predicative phrases their members are classified in terms of the main syntactic elements: subject and predicate: Day broke; The car stopped.
To avoid misunderstanding it is convenient to use the term 'syntactic element' describing phrases: subjective syntactic element, predicative syntactic element.