Is it...

The boss's car
The boss' car

Or something else

And having typed it out and stared at it for 10 minutes, I'm much in favour of the former.

yes

heh

sorry, couldn't resist.

I would go with the latter, just because three ss's in a row looks kinda silly.

The latter for me as well. The Boss' Car.

Headmistressship was the only triple sss word I could come up with.

I'm happy with silly if it's right.

My initial thinking was Boss', for the very reasons given. But everything I can find (via professor g) tells me that only mythical / relgious entities can skip the s, and that everyone else is stuck with it. Having struggled with the possessive of Davies (my surname) for 25 odd years, I thought it would be nice to know with *some certainly. And Bighard Word is no help...

I learned to use boss' when I was in grammar school, but nowadays they teach boss's. Go figure. I still tend to write it like I was l'arned.

I say, if you pronounce it "the boss car" go with 2. If you pronounce it "the bossiz car" go with 1.

As there are protocols in politics, military, computers, ets., there are similar in grammar and language. In the USA, the newspaper industry has had a tremendous influence re: rules of grammar and spelling. The use of the apostrophe in possessives has been the cause of controversy. Isn't it the Chicago Style Manual that most American newspapers recognize as their standard? When I was in college (early 1960s), most American colleges used Harbrace for routine issues and Turabian for formal papers. But the Chicago style was beginning to intrude so that today it is predominant. Wonder what is going to happen as the hard-copy newspapers are having an increasingly difficult time of holding on to their share of the media market?

BTW, I believe that boss' is pronounced as bosses as if it were spelled boss's.

That's as may be. It's not spelled as if it were pronounced as bosses.

are apostrophes spelling?

It's not spelled as if it were pronounced as bosses.

Oh, you want a "system" where things are spelt as they're writ? I see.

Claro que sí.