The presence or absence of commas with a relative clause is often important in determining which relative pronoun is chosen in a particular context.

A "non-comma clause," also known as a restrictive clause, is "restricting" reference to specific members of a class, excluding others for the moment.

• El libro que yo leo ahora es aburrido.
  • The book that I'm reading now is boring.
• Los chicos que no hablan inglés están aislados.
  • The boys that don't speak English are isolated.

By contrast, a "comma clause," also known as a non-restrictive clause, provides additional information about the persons or things mentioned:

• El libro, que yo leo ahora, es aburrido.
  • The book, which I'm reading now, is boring.
• Los chicos, quienes no hablan inglés, están aislados.
  • The boys, who do not speak English, are isolated.

The importance of this distinction for the choice of relative pronouns is discussed in the presentation.