Linguists are more unanimous in their interpretation of subordination in comparison with co-ordination. Traditionally, subordination is thought to be based on inequality of combining words: one of the components domi­nates over the rest and subordinates them when their form and position are concerned. The dominating element is called head of a subordinate word combination and may be expressed by various parts of speech.

The head of the subordinate word combination, i.e. its principal element, is also characterized by its own features that permit its identification. How­ever, there are no formal grammatical criteria that would distinguish the head element within the subordinate word combination. As a rule, the head ele­ment is identified against the background of syntactic relations as an element whose syntactic function remains unspecified on the given level of analysis. For example, in extremely expensive, the adverb extremely is treated as ad­verbial modifier, whereas the function of the adjective expensive remains outside the analysis of the word combination. As a result, the adjective is regarded as head of the word combination. Extending the word combination (i.e. introducing new subordinating units) leads to shifting the head element. For example, in extremely expensive cars, the adverb extremely is treated as adverbial modifier, the adjective expensive functions as attribute, the syn­tactic function of the noun cars within the given word combination remains unidentified, which permits to interpret the word as head. Introduction of one more subordinating element has similar effect and shifts the head again: to sell extremely expensive cars, where syntactic functions of the three com­ponents – extremely, expensive and cars – is clear but the newly introduced element to sell does not reveal its syntactic function within the word combi­nation in question and, consequently, is the head of the group.

The above mentioned test allows clarifying two issues: firstly, syntactic functions of dependent elements in a word combination back up the claim of subordinate relations between them; secondly, impossibility to identify syntactic function of an element in the group signals its dominating status. It should be borne in mind that both the head and dependent elements are analyzed without turning to a wider context.

To many researchers’ mind, in English, like in other Indo-European lan­guages, subordinate groups are more widely used than co-ordinate ones and dominate in a number of syntactic groups used in speech.

Structurally, subordinate constructions are diverse: dependent elements may be positioned on the left (so-called regressive structures): a small cotton dress; rather pretty; absolutely amazing; to bluntly deny. Dependent elements may also be placed on the right forming a progressing structure: a grain of truth, to accuse of lying, afraid of water, good at languages. There are also subordinate constructions with the head in the centre, framed on two sides by dependent elements: a winding path to the house, a thick pile of  paper, etc.

Unlike co-ordinate constructions, the number of components in subordi­nate groups is traditionally considered finite. Research, however, has proved that a word combination with a noun as head and an attribute in post-posi­tion, expressed by a prepositional group with local meaning, may have an unlimited number of components: the man in the store across the street by the bank under the bridge, etc.

So, the majority of linguists recognize the three-member opposition: “interdependence – co-ordination – subordination”. These relations are very abstract and do not characterize syntactic functions of elements but only mark their mutual status.