§ 10. Complex sentences which have two or more subordinate clauses discriminate two basic types of subordination arrangement: parallel and consecutive.

Subordinate clauses immediately referring to one and the same principal clause are said to be subordinated "in parallel" or "co-subordinated". Parallel subordination may be both homogeneous and heterogeneous. For instance, the two clauses of time in the following complex sentence, being embedded on the principle of parallel subordination, are homogeneous — they depend on the same element (the principal clause as a whole), are connected with each other coordinatively and perform the same function: When he agrees to hear me, and when we have spoken the matter over, I'll tell you the result.

Homogeneous arrangement is very typical of object clauses expressing reported speech. E.g.: Mrs. Lewin had warned her that Cadover was an extraordinary place, and that one must never be astonished by anything (A. Huxley).

By heterogeneous parallel subordination, co-subordinate clauses mostly refer to different elements in the principal clause. E.g.: The speakers who represented different nations and social strata were unanimous in their call for peace which is so ardently desired by the common people of the world.

As different from parallel subordination, consecutive subordination presents a hierarchy of clausal levels. In this hierarchy one subordinate clause is commonly subordinated to another, making up an uninterrupted gradation. This kind of clausal arrangement may be called "direct" consecutive subordination. E.g.: I've no idea why she said she couldn't call on us at the time I had suggested.

Alongside of direct consecutive subordination there is another form of clausal hierarchy which is formed without an immediate domination of one subordinate clause over another. For instance, this is the case when the principal clause of a complex multi-level sentence is built up on a merger basis, i.e. includes a subject or predicative clause.

E.g.: What he saw made him wince as though he had been struck.

In the cited sentence the comparative subordinate clause is dominated by the whole of the principal clause which includes a subordinate propositional unit in its syntactic position of the subject. Thus, the subordinative structure of the sentence is in fact consecutive, though not directly consecutive. This type of hierarchical clausal arrangement may be called "oblique" consecutive subordination; it is of minor importance for the system of subordination perspective as a whole.

The number of consecutive levels of subordination gives the evaluation of the "depth" of subordination perspective — one of the essential syntactic characteristics of the complex sentence. In the first three examples cited in the current paragraph this depth is estimated as 1; in the fourth example (direct consecutive subordination) it equals 3; in the fifth example (oblique consecutive subordination) it equals 2. The subordination perspective of complex sentences used in ordinary colloquial speech seldom exceeds three consecutive clausal levels.