a collar that will release under pressure when you are away if the dog gets caught on something. If it doesn't do that, you haven't solved the risk. It doesn't come off easily--it requires a good, sharp tug to release. Now that our dogs are older, we only find them in the yard if there's been some serious rough-housing going on (very rare) -- and when we find one, we know the collar did what it was supposed to, since rough-housing in a collar is how we lost a dog years ago (when another dog's jaw slid under the collar, the one wearing the collar rolled, creating a figure-8 around the jaw; the harder the dog on top pulled, the more it strangled the dog on the bottom).
We microchip our dogs, too, so there's a second layer of protection in case a collar and tags come off.
You can buy them on Amazon too:
. I like calling to buy them though because the customer service has been excellent.