There's a grammar thing that's been bothering me for a while now: sometimes people put commas between adjectives, and I know that the commas don't belong there but I can't explain why. A clear example is What a good, little boy! Most people would agree that comma is out of place. But why? We use commas in What a beautiful, complex, challenging book! It's one of those I-know-it-when-I-see-it things, and those always make me uneasy with grammar because what if my precious instincts are wrong?

The truth is more complicated and elegant than I had guessed: adjectives in English go in a precise order by category, which goes like this:

opinion :: size :: age :: shape :: color :: origin :: material :: purpose

and as long as the adjectives can be categorized you don't need commas between them. A discussion of the phenomenon is here; an explanatory chart is here.

What's amazing is that every native English speaker has absorbed this system and maybe one in a thousand could explain it.