What are coordinate adjectives and why should you have a comma between them?

Coordinate adjectives are adjectives that appear in sequence to modify the same noun.
- She bought the bright, shiny Volvo because it suited her tastes.
'Bright' and 'shiny' both modify 'Volvo', so they are separated by a comma. You can replace the comma with 'and' or change the order of the words, and the sentence still makes sense.
She bought the shiny and bright Volvo because it suited her tastes.
On the other hand, cumulative adjectives modify the noun in a specific order that can't be rearranged. For example:She went to the gym for a strenuous aerobic workout.
In this example, 'aerobic' modifies 'workout' while 'strenuous' modifies 'aerobic workout.' The words are not modifying the same noun, so you don't need to use a comma.