An essential relative clause provides necessary, defining information about the noun. On the other hand, non‐ essential relative clauses provide additional, non‐necessary information about the noun.

An important difference, however, between both types of clause is the use of punctuation. With non-defining relative clauses, we separate the clause with commas.

In this paper, the syntactic differences exhibited by English restrictive relative clauses vs. non-restrictive or appositive ones are accounted for by positing that restrictive operators move to C° ...

Winter is a time when it sometimes snows. Non-defining relative clauses are made in the same way. An important difference, however, between both types of clause, is the use of punctuation.