If two adjectives modify a noun in the same way, place a comma between the two adjectives. These are called coordinate adjectives.
There is a two-part test for coordinate adjectives:
(1) Can you replace the comma with the word and?
(2) Can you reverse the order of the adjectives and keep the same meaning?
If you can do both, then you have coordinate adjectives.
Correct: Did you read about Macomber’s short, happy life?
Test for Correctness: Did you read about Macomber’s short and happy life?
Did you read about Macomber’s happy, short life?
All three sentences say the same thing, so the adjectives are coordinate adjectives and separated by commas in the original.
If the paired adjectives fail the two-part test, then no comma is used. This shows that they must remain in a certain order to make sense. These are called cumulative adjectives.
Incorrect: The former, overweight woman told us how she lost fifty-five pounds.
Test for Correctness: The former and overweight woman…
(Makes no sense)
The overweight, former woman…
(A former woman? At best the meaning is changed.)
Clearly, no comma is needed for these cumulative adjectives.
Correct: The former overweight woman told us how she lost fifty-five pounds.
A device to help remember this punctuation rule is to keep in mind a common expression like Christmas tree or fire truck. We say, “green Christmas tree,” but not “Christmas green tree.” We say, “red fire truck,” but not “fire red truck.” Such cumulative expressions take no comma.
In some cases, it may be possible to change the first adjective to an adverb if it still makes sense by modifying just the second adjective. This will not work if the adjective must modify both the adjective and noun that follows it.
Correct: The formerly overweight woman told us how she lost fifty-five pounds.
However, that would not work for a sentence like the following:
Correct: The former secret agent had to change her identity.
In a case like this last one, the first adjective is clearly modifying the whole phrase secret agent.