Earlier in this lesson, you learned that a subordinate clause at the beginning of a sentence is followed by a comma,but a subordinate clause any other place in the sentence is not set off by a comma. This is true only if the clause is an essential clause. In some sentences,a clause cannot be omitted without changing the basic meaning of the sentences. Omitting such a clause changes the meaning of the sentence or makes it untrue. Such a clause is called an essential or restrictive clause.