What's the difference between a coordinate adjective and a hierarchical adjective?

Coordinate adjectives work together to modify a word; that is to say, they have the same function. For example, in the phrase the spooky, mysterious house, 'spooky' and 'mysterious' modify 'house' to the same degree.

This can be tested in two ways. Firstly, if you can put and in between the adjectives in place of a comma and it still makes sense, they are coordinate adjectives. Secondly, if you can switch the two adjectives around and still have the same meaning, they are coordinate adjectives. Testing our example, we can see that the spooky and mysterious house and the mysterious, spooky house both make sense.

Hierarchical adjectives do not have equal rank to the other adjectives in the phrase, i.e. the adjectives exist in hierarchy. This means that changing the order of the words or placing the word and between them would not make sense. For example, the cold December wind could not be the cold and December wind or the December, cold wind. Hierarchical adjectives are used without a comma.

Sometimes we see both hierarchical and coordinate adjectives in the same sentence. We can use the two tests to understand where to place the commas.

For example, take the sentence I'd really like a hot delicious pepperoni pizza. If we introduce "and," this word order doesn't make sense: I'd really like a pepperoni and delicious hot pizza; whereas I'd really like a hot and delicious pepperoni pizza does make sense. This means the comma belongs between the two coordinate adjectives, hot and delicious. Pepperoni is the hierarchical adjective. So, the correct version of this sentence is: I'd really like a hot, delicious pepperoni pizza.