When two adjectives both describe the same noun, we put a comma between the adjectives.
That is a big house.
That is a beautiful house.
That is a big, beautiful house.
If we can say That is a big and beautiful house, then we can say That is a big, beautiful house. The comma takes the place of the word and.
More examples where the comma could be replaced with the word and:
I crept into the dark, scary room.
I found a big, heavy box in the basement.
My birthday was a wild, crazy day.
Do not use a comma between adjectives if you can’t use the word and between the adjectives.
This sentence has two adjectives but we do not put a comma between them:
Martha wore an ugly brown dress.
We wouldn’t say Martha wore an ugly and brown dress. Therefore, don’t use a comma between the adjectives.
We couldn’t get across the busy main street.
No comma here because we can’t say the busy and main street.
Place a comma between two adjectives modifying the same noun when you could also place the word and between the adjectives.
▶Now you try it. Decide if this sentence needs a comma between the two adjectives. I miss the sun on gray cloudy days. Did you put in a comma? Good for you!