Coordinate adjectives are adjectives that appear in sequence with one another to modify the same noun. For example, the adjectives in the phrases bright, sunny day and dark and stormy night are coordinate adjectives. Coordinate adjectives are usually separated with either commas or and, and and always comes before the final adjective.
Some sequential pairs or groups of adjectives that modify single nouns are not coordinate. For example, in the phrase harsh verbal warning, harsh and verbal are not coordinate adjectives because harsh modifies the phrase verbal warning. If you ever have difficulty deciding whether a pair or group of adjectives is coordinate, try inserting and between them. If and would work, the adjectives are coordinate and hence do need a comma.
The adjectives highlighted in the following sentences are coordinate because they modify their nouns in a logically parallel way:
Forecasters warned of another day of hot, windy conditions across Southern California on Sunday. [Los Angeles Times]
In addition, their breathtakingly cruel and callous actions also led to a tribute plaque. [Liverpool Echo]
Obama stands accused of giving stuffy, cliche-ridden graduation speeches. [Standard-Examiner]
And these adjective pairs are not coordinate because they modify their nouns in different ways:
Amazon prepping multiple wallet-friendly tablets. [CNET]
With this in mind, I wanted to design a screen to identify dirt-cheap smaller companies. [Seeking Alpha]
The Red Wings are demonstrably a tough hockey team. [Detroit News (link now dead)]