Should I use a comma between two adjectives?
Adjectives are words that modify a noun like "old", "red", "greasy". They should really be called "adnouns" to be consistent with "adverbs." Sadly, they're not, but that's a good way to think of them.
You should use a comma between two adjectives when they are coordinate adjectives. Coordinate adjectives are two or more adjectives that describe the same noun equally.
With coordinate adjectives you can put "and" between them and the meaning is the same. Similarly, you can swap their order.
For example: The long, metal pole. "Long" and "metal" are both adjectives that describe the noun "pole": I could write "long pole" or "metal pole". I could also write "long and metal pole" and "metal and long pole" and "metal, long pole". They might sound slightly strange but the meaning is retained.
Cumulative adjectives build on each other and cannot be re-ordered or split with "and," for example: bright yellow jacket. It can't be "bright and yellow jacket" or "yellow bright jacket".
Because coordinate adjectives are equally important, they are separated by a comma. Cumulative adjectives aren't equally important and so they are not separated by a comma. Only use a comma to separate two adjectives if you could also write "and" there and keep the same meaning.
- Correct: The long, metal pole stuck out of the ground.
- Incorrect: The long metal pole stuck out of the ground.
- Correct: The bright yellow jacket looked amazing.
- Incorrect: The bright, yellow jacket looked amazing.