III. A subordinate clause may depend on another subordinate clause.
- The horse shied when he saw the locomotive. [The subordinate clause depends upon the independent (main) clause.]
- The horse shied when he saw the locomotive, which was puffing violently. [The second subordinate clause depends upon the first, being an adjective modifier of locomotive.]
In such cases, the whole group of subordinate clauses may be taken together as forming one complex subordinate clause.
Thus, in the second example, when he saw the locomotive, which was puffing violently may be regarded as a complex adverbial clause modifying shied, and containing an adjective clause (which was puffing violently).
506. From the principles summarized in [§ 505], it appears that—
Clauses (like sentences) may be simple, compound, or complex.
1. A simple clause contains but one subject and one predicate, either or both of which may be compound ([§ 451]).
2. A compound clause consists of two or more coördinate clauses ([§ 454]).
3. A complex clause consists of at least two clauses, one of which is subordinate to the other.
507. The unit in all combinations of clauses is clearly the simple sentence, which, when used as a part of a more complicated sentence, becomes a simple clause.