What is the proper grammer for possesive plural?

Question

for example, if I have three cars each with radios and I want to refer to the radios, do I use an apostophe at the end ?

ie cars' radios

Answer 1

Yes you do!

You write cars's but drop the final s.
So- cars'.

It's the same as people's names that end in s [like mine]
Francis' cars.

PS just to comment on the other contributors.
The cars DO own the radios. The radios DO belong to the cars. Just like a handle belongs to a door.

My answer remains the correct one.

Answer 2

Usually the item is referred to as a "car radio." Thus car is an adjective to describe the type of radio you are describing. Cars don't own anything, people do.

"The car radios I own are all high-end systems."

Another way of writing the sentence: "Each of my three vehicles is equipped with a fancy car radio."

Of course, in the US many people would now call such devices "sound systems" which is a more contemporary term than the old-fashioned car radio.

Example: "I'm proud to say each of my cars is equipped with a state-of-the-art sound system."

Answer 3

No apostrophe. The radios do not possess anything, the cars do. The apostrophe at the end of cars is correct.