Rant on the Insidious Appearance of the Penultimate Comma

Yes, I know this is a blog on mental health, not on grammar.  However, a phenomenon has crept into the written English language that threatens my mental health, since it causes me to scream every time I encounter it.  It is the Penultimate Comma, which is a comma that appears between two modifiers preceding a noun.  It looks like this, when used properly:

“A big, black dog.”  (You could also correctly write “A big black dog.”)

When used improperly, it looks like this:

“A white, Cadillac convertible.”

What’s the difference?  It’s very simple.  If you can hypothetically insert the word “and” between the two modifiers, you can substitute a comma for the “and.”

As in: “A big and black dog.”  You wouldn’t necessarily say it that way, because it sounds awkward, but it’s grammatically correct to do so.

However, to say: “A white and Cadillac convertible” sounds bizarre, if not ridiculous.  

I know from whence this grammatical misconception arose: school children of this, and sometimes the previous, generation have been taught that if you can say “A white convertible” and also “a Cadillac convertible,” then you should go ahead and insert a comma between “white” and “Cadillac.”  This is wrong.

Why?  Because, dear readers, it sounds utterly absurd.  That is why.  If you read it out loud, placing the pause of the comma where it is written, you will see.

Another incorrect example:  “An expensive, Tudor house.”  No, no, no!  Yes, it is an expensive house, and it is also a Tudor house,  but it is “an expensive Tudor house” and that’s that.

Another correct example:  “An expensive, garish negligee.”   Why?  Because you can say “An expensive and garish negligee.”  Very simple.

How do I come by the audacity to write this vituperous essay on the Penultimate (next-to-last) Comma?  It is simply a product of thirty years, more or less, of editing various  book manuscripts and hundreds of medical and scientific papers, as well as a couple of dissertations.  I learned by Experience.  Period.