• Interrupting Phrase
• Nonrestrictive Relative Clause
• Exercise in Punctuating Nonrestrictive Elements
• Guidelines for Using Commas Effectively

• How to Build Sentences with Appositives
• Modifier
• Restrictive and Nonrestrictive Adjective Clauses
• Subordinate Clause

Examples and Observations:

• "Audrey Wishard McMillan, who was born in India, was the daughter of Americans living abroad and was educated in a school for children of American missionaries."
  (Judy Green and Jeanne LaDuke, Pioneering Women in American Mathematics. American Mathematical Society, 2009)
• "Human beings, who are almost unique in having the ability to learn from the experience of others, are also remarkable for their apparent disinclination to do so."
  (Douglas Adams, Last Chance to See. Harmony Books, 1991)
• "As the one lane became two, Ben moved from the left lane towards the right one, and the couple, who first met at high school, were chatting easily. And then Ben, who was sitting on the speed limit of 60 kilometres an hour, started to grow a bit edgy. He told Renee about the idiot he could see in his rear-vision mirror who was driving too fast."
  (Madonna King, Catalyst: The Power of the Media and the Public to Make Change. University of Queensland Press, 2005)

• "Many other technologies resulted from serendipity. A well-known example is penicillin, which was discovered by accident by Sir Alexander Fleming.
  (Everett M. Rogers, Diffusion of Innovations, 5th ed. Free Press, 2003)
• "The book was a life of Brahms, which had been standing askew on one of the shelves here and which the dampness had left permanently misshapen."
  (David Markson, Wittgenstein's Mistress. Dalkey Archive Press, 1988)
• "Samsø, which is roughly the size of Nantucket, sits in what’s known as the Kattegat, an arm of the North Sea. The island is bulgy in the south and narrows to a bladelike point in the north, so that on a map it looks a bit like a woman’s torso and a bit like a meat cleaver."
  (Elizabeth Kolbert, "The Island in the Wind." The New Yorker, July 7, 2008)
• "Health sciences, computer science, engineering and business--fields that have tended to attract a somewhat greater proportion of moderates and conservatives--have grown in importance and size compared with the more liberal social sciences and humanities, where many of the bitterest fights over curriculum and theory occurred."
  (Patricia Cohen, "The ’60s Begin to Fade as Liberal Professors Retire." The New York Times, July 4, 2008)
• Restrictive and Nonrestrictive Relative Clauses (RC)
  "Clauses that modify nouns, such as the one in (4), are referred to as relative clauses because the noun they modify (stories in this case) plays a role (has a function) in the RC. The RC is related to the noun by means of which:

  (4) The stories [which he repeats often] are boring.

  The element that connects the noun and the clause, i.e. which in (4), is called a relative pronoun. In (4), the relative pronoun functions as the direct object of repeat. . . .
  
  "RCs are usually divided into restrictive as in (4) and non-restrictive, as in (5) and (6):

  (5) Hillary Clinton, who just returned from a trip to Cuba, intends to write a book.
  (6) Queen Elizabeth the first, who was born in 1533, was the last sovereign of the house of Tudor.

  The reason we discuss the difference between restrictive and non-restrictive clauses is that the use of one over the other has grammatical (and possibly other) consequences."
  (Elly van Gelderen, An Introduction to the Grammar of English, rev. ed. John Benjamins, 2010)
• Restrictive and Nonrestrictive Modifiers
  "Not all participial phrases are restrictive. Sometimes the referent of the noun is already identified, so a modifier isn't necessary. In such cases, the purpose of the modifier is simply to comment on or to add information about the noun, not to define it. Such modifiers are called nonrestrictive modifiers:

  My mother, sitting by the window, is talking to herself.

  In this sentence the noun phrasemy mother is already specific; it has only one possible referent. Sitting by the window simply adds a detail of information."
  (Martha Kolln, Rhetorical Grammar: Grammatical Choices, Rhetorical Effects, 3rd ed., Allyn and Bacon, 1999)
• Punctuating Nonrestrictive Clauses
  "Nonrestrictive relative clauses . . . do not restrict the reference of the noun. They are also usually set off by commas in writing, and you can also usually detect 'comma intonation' in a speaker's voice . . ..

  restrictive
  The paint which Mary bought at the hardware store was bright red.
  
  nonrestrictive
  The paint, which Mary bought at the hardware store, was bright red.

  The restrictive relative clause which Mary bought at the hardware store, limits which paint we're referring to, namely to, paint which Mary bought at the hardware store. The nonrestrictive relative clause, on the other hand, does not restrict the reference of the noun paint; it is not information that distinguishes the paint from other paint. That Mary bought this paint at the hardware store is simply incidental information."
  (Anne Lobeck and Kristin Denham, Navigating English Grammar: A Guide to Analyzing Real Language. Wiley-Blackwell, 2014)
• Restrictive and Nonrestrictive Elements: That and Which
  "Ordinarily, the conjunction 'that' would introduce a restrictive clause. Nonrestrictive: This is a baseball, which is spherical and white. Restrictive: This is the baseball that Babe Ruth hit out of the park after pointing at the fence in Chicago. The first ball is unspecific, and that sentence requires a comma if the writer wishes to digress into its shape and color. The second ball is very specific, and the sentence repels commas."
  (John McPhee, "The Writing Life: Draft No. 4." The New Yorker, April 29, 2013)