Where to draw the line separating non-essential and essential relative clauses

Question

On Purdue Owl the definition of an essential clause is "restricting the meaning of a modified term" and "if the essential meaning of a sentence changes when you leave out the element or put it somewhere else then the element is essential."

On softschools.com: "An essential adjective clause is one that is needed for the sentence to make sense."

On Grammar bytes: limits a general, ambiguous noun. The essential clause tells the reader which one of many the writer means.

Its examples:

Essential: The man who ordered another double anchovy pizza claims to have a pet dolphin in his backyard pool.

Non-essential: Mr. Hall, who ordered another double anchovy pizza, claims to have a pet dolphin in his backyard pool.

If I replaced "the" with "this" in the first sentence wouldn't that make "man" less ambiguous. Less enough to make the essential clause non-essential?

This man, who ordered another double anchovy pizza, claims to have a pet dolphin in his backyard pool.

Or do "the" and "this" just modify the noun and don't affect whether the noun is vague enough to need an essential clause.

Except in one of purdue's examples:

Nonessential: Company managers, seeking higher profits, hired temporary workers to replace full-time staff. (phrase)

I agree that "seeking higher profits" isn't essential to understand the intended point of the sentence but isn't "Company managers" vague. Is there a more absolute way of defining essential and non-essential clauses? Or are all the definitions wrong and sometimes it's up to the writer to choose between what to keep essential and non-essential like in this example:

The man, who ordered another double anchovy pizza, claims to have a pet dolphin in his backyard pool.

or this one

The man, who I find odd, who ordered another double anchovy pizza claims to have a pet dolphin in his backyard pool.

Answer 1

Okay, here goes. I will analyze each of your nine sentences.

1) This is perfectly grammatical and sensical. As you said, "which happens to be glowing red" is a relative clause.

2) Once again, this sentence is grammatical, just the previous one without the relative clause.

3,4) Also grammatical, but with a different meaning: The toaster is the cheapest (among an assortment of items).

5) The indirect quote is formed properly. This sentence has no issues.

6) This sentence could be used to explain to a third party why you chose to buy this particular toaster. It is grammatically and logically sound.

7) Your comment on which relative pronoun to use is stylistic, and I would keep it the way it is. "Which happened to..." just sounds more natural than "that happened to..."

8) The second use of the word "toaster" is redundant, not ambiguous. The sentence is grammatical and logical, but I can not see it being used well.

9) This just changes the indirect quote to a direct one without affecting the meaning of the sentence.

Answer 2

Changing the to this doesn’t make the vague noun any less vague. Even though this sounds like it demonstrates a specific man, the reader still doesn’t know which man you mean unless the previous sentence stated a specific name. If the sentence is by itself without context and you choose to use a relative clause to describe a vague noun that clause should be essential. But not must.

Left alone with no context these 2 sentences have different meanings:

The man, who ordered another double anchovy pizza, claims to have a pet dolphin in his backyard pool.

The man who ordered another double anchovy pizza claims to have a pet dolphin in his backyard pool.

The first with commas means: that man claims to have a pet dolphin in his backyard pool, and he just happened to order another anchovy pizza, that he did is incidental.

The second without means: specifically the guy who ordered another double anchovy pizza claims to have a pet dolphin in his backyard pool.

For Grammar bytes's definition to be correct the context has to be vague. The sentence before "anchovy guy" might read:

Lois was at a get together with a lot of people. The man who ordered another double anchovy pizza claims to have a pet dolphin in his backyard pool.

If the sentence before read:

Lois was at a get together with her woman's only book club and Peter. The man, who ordered another double anchovy pizza, claims to have a pet dolphin in his backyard pool.

Scrooble made it clear when he answered the first version of my question

The toaster, which happens to be glowing red, is the cheapest.

The toaster is the cheapest (among an assortment of items).

Even this sentence is ok with a non-essential clause.

The thing, which happens to be glowing red, is the cheapest.

Because thing could be the only object in the store that only sells plants (living thing) except for the one object (thing). No context is given the vague noun with a non-essential clause is NOT wrong. NOT wrong as in putting a non-essential clause where an essential clause should be, Grammar has nothing to do with wrong or right.

My definition of an essential and non-essential clauses

Essential clause/Restrictive clause/Defining clause: A Relative clause that contains information required to understand the intended point of the sentence. If the essential clause is removed the intended point of the sentence is lost even if the sentence is still grammatical; Never set off by a comma or two

Non-essential clause/Non-restrictive clause/Non-defining clause: A Relative clause that doesn’t contain information required to understand the intended point of the sentence; always set off by a comma or two.

BONUS If your still here:

Another Grammar bytes example: As we sped through the neighborhood, we spotted crows eating French fries tossed on the road. They did not fly to a tree as we expected. The birds, which never showed fear of the vehicle, watched as we swerved around them.

If you changed the last sentence to:

The birds that never showed fear of the vehicle, watched as we swerved around them. It sounds like there may be some birds that didn’t watch them swerve around, and they died. If that was the case just write:

some birds, which never… around them. The others weren’t so lucky.

So I guess the lesson here is use an essential clause when you want to say something more specific about a noun and a non-essential clause when you want to say something incidental.