1. A sentence containing a single statement is called a Simple Sentence.
2. A sentence containing more than one statement is called a Compound Sentence, and each single statement in it is called a Clause.
a. If one statement is simply added to another, the clauses are said to be Coördinate. They are usually connected by a Coördinate Conjunction (§ 223. a); but this is sometimes omitted: -
b. If one statement modifies another in any way, the modifying clause is said to be Subordinate, and the clause modified is called the Main Clause.
This subordination is indicated by some connecting word, either a Subordinate Conjunction (§ 223. b) or a Relative: -
A sentence containing one or more subordinate clauses is sometimes called Complex.
NOTE: A subordinate clause may itself be modified by other subordinate clauses.