A clause is the smallest grammatical unit that can express a complete idea. A typical clause consists of a subject and a predicate.

The predicate is typically a verb phrase: a verb together with any objects and other modifiers.

Types

Independent

An independent clause (or main clause) is a clause that can stand by itself as a simple sentence. An independent clause contains a subject and a predicate. It makes sense by itself and therefore expresses a complete thought.

He has three tiger cubs.

Subordinate

A subordinate clause is reliant on the appearance of a main clause. It depends on the main clause and is a dependent clause. It does not express a complete thought on its own. Subordinate clauses follow this format:

• SUBORDINATE CLAUSE (comma) MAIN CLAUSE.
• MAIN CLAUSE (no comma) SUBORDINATE CLAUSE

In Oklahoma, {note comma} ~~it's perfectly legal to own a tiger cub.
It's perfectly legal to own a tiger cub in Oklahoma.

Relative

A relative clause will begin with a relative pronoun (who, whom, whose, which, or that) or a relative adverb (when, where or why).

Essential relative clauses do not require commas. A relative clause is essential when you need the information it provides. The sentence would not make sense or would be awkward without the relative clause.

A tiger who eats expired meat will develop bad breath.

In this sentence, tiger is nonspecific. To know which instance of "tiger" we are talking about, we must have the information in the relative clause. Thus, the relative clause is essential and requires no commas.

A nonessential relative clause can be removed from the sentence without affecting sentence structure or content. Try reading the sentence without the clause in question. If the sentence is awkward or unintelligible without the clause, no commas are needed.

A tiger, who eats expired meat, has developed bad breath.

If, however, we revise the sentence and choose more specific words instead, the relative clause becomes nonessential and does require commas to separate it from the rest of the sentence. In this sentence, the dog has been specified. Notice how we could leave the nonessential clause out, and the sentence would still make sense: "A tiger has developed bad breath."