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.