Comma Rule: Separate Adjectives Modifying a Noun

I was editing my book today and had to look up a comma rule. When this happens, I try to jot my findings down and then share it with someone. This is kind of a comma rule within a comma rule (those are always fun). I might have known this rule years ago, but having two instances of comma issue popping up twice in one paragraph, I thought I should look up the rule just to make sure I was doing it right.

To explain the rule within the rule, we first must look at the base rule, which is this: You use a comma to separate two or more adjectives that modifies a noun.

Simple enough. Here’s an example of that in action: The ingenuous, fervent preacher.

“Ingenuous” and “fervent” are adjectives modifying the noun “preacher.” Since they are both modifying the same noun, put a comma between them to distinguish them.

OK, here’s the second rule that sometimes pops up with the above comma rule: You shouldn’t put a comma between two adjective if the last adjective acts as a unit with the noun.

Here’s what I mean. I’ll give two examples:

1) I found that it was a half-spherical glass dome.

2) The bright moon glow was stunning.

Those two lines actually were the lines in my book that brought me to look up the rule. In the first example, the adjective “glass” acts as a unit with “dome,” so there is no comma between “half-spherical” and “glass.” Same with #2’s “bright” and “moon”; there’s no comma between those two words because “moon” is a unit with “glow.”

Now, how can you tell if the adjective is a “unit” with the noun? There’s an easy way to figure this out. If you insert “and” where the comma would go and it doesn’t sound awkward, then it’s not a unit (ex. The ingenuous and fervent preacher). If it does sound awkward (ex. The bright and moon glow was stunning) then you know it’s a unit, which means you don’t need a comma between the two modifying adjectives.