Examples:
* No comma: They increased production, thereby lowering the cost per unit. (The phrase "thereby lowering the cost per unit" is essential to the sentence's meaning.)
* Comma: They increased production, thereby increasing their profits, but also leading to higher unemployment. (The phrase "but also leading to higher unemployment" is a non-restrictive clause, providing additional information.)
Here's a breakdown:
* Restrictive clause: These clauses are essential to understanding the sentence's meaning. They cannot be removed without changing the sentence's meaning. You don't use a comma before them.
* Non-restrictive clause: These clauses provide additional information that is not essential to the sentence's meaning. They can be removed without changing the sentence's meaning. You use a comma before them.
If you're unsure whether a clause is restrictive or non-restrictive, consider if you can remove it without changing the meaning of the sentence. If you can, it's non-restrictive and you should use a comma before it.