Note: I’m addressing this issue in part because a colleague informed me that it’s popped up in regards to my books on Reddit. If anyone comes across those questions, feel free to direct people here.

Among the simplest and most straightforward grammatical rules students studying for the SAT or ACT often learn is two commas are often used to signal non-essential information: words, phrases, and clauses that are not central to the essential meaning of a sentence, and that can be crossed out without affecting its basic grammatical structure.

The problem, of course, is that commas can be tested in many ways, and that two commas can be present in a given section for numerous reasons. Now, much of the time, two commas in an underlined section will in fact signal non-essential information, but if you’re aiming for a very high Writing/English score on the SAT or ACT, you also need to understand when this is not the case. (To read about information that is non-essential click here.)

Consider the following sentence:

Some of the most powerful telescopes in the world now peer across the vast, empty distances of space, watching for the faintest dip of light or wobble that could suggest the presence of another world.

If you cross out the information between the commas, however, the sentence no longer makes sense.

Some of the most powerful telescopes in the world now peer across the vast…watching for the faintest dip of light or wobble that could suggest the presence of another world.

Because the sentence doesn’t hold together this way, it might seem reasonable to conclude that two commas aren’t necessary. But in fact, they are necessary — they’re just there for other reasons.

The first comma is correct because the adjectives vast and empty could also be separated by and (vast and empty), and the second comma is correct because it divides the first, independent clause from the dependent (participial) phrase that follows. So even though there are two commas, the essential vs. non-essential issue is completely moot.

Let’s look at another example, test-style this time:

Some of the most powerful telescopes in the world now peer across the vast, empty, distances of space, watching for the faintest dip of light or wobble that could suggest the presence of another world.

A. NO CHANGE
B. vast empty, distances
C. vast empty distances,
D. vast, empty distances

This is a little trickier. The word empty is surrounded by commas, and if it is crossed out, the sentence still makes sense:

Some of the most powerful telescopes in the world are now peering across the vast…distances of space, watching for the faintest dip of light or wobble that could suggest the presence of another world.

As a result, it might seem acceptable to place commas around empty. Unfortunately, though, that’s not the case.

In this example, the section in question involves adjectives (vast, empty) modifying a noun (distance) — a construction that has absolutely nothing to do with essential vs. non-essential information.

The fact the sentence still makes sense when empty is eliminated is purely incidental; it does not mean that the word can or should be treated non-essentially. If you want a “rule,” you can keep in mind that single non-essential words are pretty much always transitional words like however, therefore. That said, in a situation like this, it’s really more important that you be able to recognize the bigger picture of what’s being tested.

To answer this question, you must apply a rule entirely unrelated to non-essential clauses, namely that no comma should be used between an adjective and the noun it modifies. So in fact, A is wrong for the same reason as B. On the other hand, D omits the unnecessary comma between the adjective and the noun, and so it is correct. (C is wrong because the commas after distances improperly places a comma before the preposition of.)

So, the moral of the story:

Learning grammar is not just a matter of memorizing rules and then applying them as literally as possible. It is also about understanding when and how those rules should be applied, and about recognizing that different contexts call for different types of applications. The reality is that sometimes rules may appear to conflict, and you can’t memorize every possible scenario in which rule x wins out over rule y or vice versa. There is a point where grammar and logic meet, and sometimes getting the right answer involves thinking (yes, thinking) about what makes the most sense in a given situation. Blunt application of rules can get you pretty far, but to get all the way, you have to be flexible too.