Commas with Coordinate Adjectives

Look at 'a long, hot summer.' Long and hot each describe summer on their own, so they're coordinate adjectives, equal partners separated by a comma. Now look at 'three old stone houses.' You can't swap the order to 'stone old three houses' and you can't say 'three old and stone houses' - old modifies stone houses as a unit, building one description in a fixed sequence. That's cumulative stacking, and stacked adjectives never take commas.

The test is mechanical: try sliding 'and' between the two adjectives, or try swapping their order. If both moves still sound natural, the adjectives are coordinate and need a comma. If either move sounds wrong, they're cumulative and need no comma. 'Quiet and efficient assistant' works fine, so quiet and efficient get a comma. 'Stone and old houses' doesn't work, so stone and old get none.

This sits next to the rest of the punctuation unit because it asks the same underlying question every comma rule asks: is this mark joining two equal things, setting off an aside, or doing nothing at all? Coordinate adjectives are the 'joining two equal things' case, structurally the same logic as commas joining two independent clauses or separating items in a list. Misreading a cumulative string as coordinate is the single most common wrong answer choice on these questions, so the and-test and the swap-test are worth running every time, not just when the sentence sounds natural.