Non-defining relative clauses, also known as non-restrictive relative clauses or non-essential relative clauses, provide additional information about a noun in a sentence without changing the essential meaning of the sentence. These clauses are set off by commas and can be removed from the sentence without altering the main message.

Characteristics of Non-Defining Relative Clauses:

1. Commas: Non-defining relative clauses are set off by commas before and after the clause. The use of commas distinguishes non-defining relative clauses from defining relative clauses, which do not require commas.

2. Additional Information: Non-defining relative clauses provide extra details about the noun they refer to, adding information that is not necessary for the sentence's core meaning. The information provided by these clauses is non-essential and can be omitted without changing the essential meaning of the sentence.

3. Relative Pronouns: Non-defining relative clauses are introduced by relative pronouns such as "who," "whom," "whose," "which," or "that." These pronouns connect the clause to the noun it modifies.

Examples:

1. My sister, who is a doctor, lives in London.
- The non-defining relative clause "who is a doctor" provides additional information about the noun "sister." The sentence would still make sense without this clause.

2. The Eiffel Tower, which was built in 1889, is a famous landmark in Paris.
- The non-defining relative clause "which was built in 1889" gives extra information about "The Eiffel Tower." The sentence's core meaning remains intact without this clause.

Key Points:

- Non-defining relative clauses provide additional, non-essential information about a noun in a sentence.
- They are marked by commas before and after the clause.
- These clauses can be removed without changing the essential meaning of the sentence.
- Non-defining relative clauses are introduced by relative pronouns like "who," "whom," "whose," "which," or "that."

Non-defining relative clauses are composed of a relative pronoun, a verb, and optional other elements such as the subject or object of the verb. Commas or parentheses are always used to separate non-defining relative clauses from the rest of the sentence.