Recently, in a column about other things, we asked whether you needed a comma in the phrase “the large blue box.” The answer is that, most of the time, you do. But, depending on what the sentence intended to say, you may not.

If you were speaking of all the blue boxes and wanted to point out the large one, you would not want a comma, because, in this case, “large” is actually modifying “blue box.” But if there were all sorts of boxes, and you wanted to point out the one that was both blue and large, you would want a comma. In the first case, the adjectives are “cumulative,” meaning they don’t separately modify “box”; in the second, the adjectives are “coordinate,” meaning they do separately modify “box.”

It’s usually pretty easy to tell the difference, once you know the tricks. The Chicago Manual of Style has a fairly clear explanation:

As a general rule, when a noun is preceded by two or more adjectives that could, without affecting the meaning, be joined by and , the adjectives are normally separated by commas. Such adjectives, which are called coordinate adjectives, can also usually be reversed in order and still make sense. If, on the other hand, the adjectives are not coordinate—that is, if one or more of the adjectives is essential to (i.e., forms a unit with) the noun being modified—no commas are used.

Take the sentence “He was a lazy unmotivated college student.” You can say “He was a lazy and unmotivated college student,” but you can’t say “He was a lazy and unmotivated and college student.” So you need a comma between “lazy” and “unmotivated,” but not between “unmotivated” and “college.”

The other test is to switch the positions of the adjectives. For our slothful student, you can say “He was an unmotivated and lazy college student, ” but just try putting “college” anywhere else but before “student.”

It’s clear that “college” modifies “student,” and that “unmotivated” and “lazy” modify “college student” as a unit.

Garner’s Modern American Usage says that “writers often include the comma when it isn’t necessary,” as in “a permanent, $500 per-child tax credit.”

Perhaps as frequently, though, writers do not include the comma, maybe because sometimes it’s hard to tell whether one would use “and” between adjectives, or maybe they just don’t know any better. Especially when the adjectives are very short, as in “the large blue box,” it may not be clear that those need to be coordinated with a comma.

Usage and grammar guides may not help. Some say that a phrase like “blue wool sweaters” should not take a comma, because “It would sound illogical to say ‘blue and wool sweaters’ or ‘wool blue sweaters.’” Chicago notes that phrases like “red brick house” do not take a comma because “they have no logical connection in sense (a red house could be made of many different materials).” Both ignore the possibility that a writer wants to emphasize that a sweater is both blue and wool, or that the house made of brick shelters a socialist commune. (OK, maybe that last bit is stretching it …)

You might think that the terms should be reversed: After all, two adjectives modifying something separately create a cumulative description, and it takes some coordination to recognize that one of those adjectives might be locked tighter to the noun than the other. So don’t try to remember “coordinate” or “cumulative.” Just try to remember the “and” part, or the old switcheroo.

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Merrill Perlman managed copy desks across the newsroom at The New York Times, where she worked for 25 years. Follow her on Twitter at @meperl.