Comma after nonrestrictive adverbial (dependent) clause at the end of the sentence

Question

I am confused whether comma is required after adverbial (dependent) clauses at the end of the sentence (and the difference, if any, between restrictive and non-restrictive adverbial clauses).

The internet and various style manuals seem to give conflicting advice, so I am not sure what is true or, at least, what is prevalent.

According to Purdue University (Purdue Owl), no comma is needed, except cases on contrast, when dependent clause follows independent clause, presumably because they are all treated as restrictive.

Example:

She was late for class because her alarm clock was broken.

Yet Gregg Reference Manual, 10th Edition and CMOS 16th edition both state that comma is required if the dependent clause following an independent clause is non-restrictive in meaning, but no comma is required if the clause is restrictive.

Gregg gives the following examples of restrictive and non-restrictive dependent clauses (dependent clauses in italics - notice the comma in the second example):

Restrictive: His faxed response came after you left last evening.

Non-restrictive: His faxed response came this morning, after the decision had been made.

While I see the logic and the difference, I would argue that the second example is also restrictive depending on what is being restricted. And, arguably, Purdue Owl would punctuate the second sentence, based on the advice given, without the comma.

So what holds true here?

Is there a majority-accepted rule (or, at least, majority position) around restrictive / non-restrictive adverbial clauses, or is this merely a stylistic / subjective preference? Could all clauses be punctuated as restrictive, aside from some obvious exceptions below?

I know there are some exceptions with negatives clauses (Not because, but because, etc.). And also when the clause is clearly an afterthought (I can do this, if you wish). But in cases like the above where either interpretation is possible what is the prevalent editorial position?

Could I argue that the sentences below should also be punctuated without comma on the ground of stylistic / subjective preference - rationale in brackets?

I will see you at 9.35 pm after the party has finished. ("9.35" describes what time; "after the party has finished" describes circumstances - both restrictive; no comma.)

The plane landed after 10-hour flight at 10 am. ("after 10-hour flight" describes circumstances; "10 am" describes time - both restrictive; no comma.)

Thanks a lot in advance (editing a piece of work and cannot go to sleep as these adverbial constructions keep coming up).

Answer 1

Is there a majority-accepted rule (or, at least, majority position) around restrictive / nonrestrictive adverbial clauses, or is this merely a stylistic / subjective preference?

All punctuation rules are stylistic, and most manuals of style recognize that rules have exceptions and are flexible enough to accommodate an author's judgment. Thus you may argue just about anything you wish. But bear in mind that you may mislead your readers. When you write nonrestrictively

I went to the airport to pick up my sister, Ellen.

your readers will infer nothing about additional siblings. On the other hand, when you write restrictively

I went to the airport to pick up my sister Ellen.

your readers will infer that you have at least one other sister not named Ellen.

This doesn't matter much when the semantics of the sentence states or implies the information.

In the sentence

I will see you at 9:35 pm after the party has finished.

the adverbial clause is nonrestrictive because the clock time has already defined the time of the assignation. If you leave out the comma, nobody will assume that you mean any other 9:35P, multiples of 24 hours before or after.

Likewise

The plane landed after a 10-hour flight at 10 am.

will likely be interpreted as

The plane landed at 10 am after a 10-hour flight.

and that if you had another 10-hour flight in mind, one that landed at some other time, your readers will assume that you would have mentioned that.

Although you're entitled to your own style, be aware that by banishing the nonrestrictive comma, you give up some concision. As you pointed out, in the sentence

His faxed response came this morning after the decision had been made.

the clause "after the decision has been made" may be restrictive, i.e., it may define the time of the morning, distinguishing the time of the fax's arrival from other events of that morning, as in

His faxed response came this morning after the decision had been made but before the press had been informed.

Or it may be nonrestrictive, in the case that nothing else if note happened that morning:

His faxed response came this morning, after the decision had been made.

A rational stylistic argument may be made that it is better not to rely on so slight a mark as the comma to convey important information. In that case you may have to add words or shift them (e.g., by making the nonrestrictive clause an introductory one). If you're not writing for yourself, however, you may writing for an editor (or grader) who can impose a manual of style upon you.