Shackles, by definition, are a device used to tighten or couples. Now, it is a very generic definition. Many different things can then be classified as one. However, the most common and well-known may be found on you now, or rather, on your keys, purse or backpack.
When you have a key chain and is not connected with a ring, it is most likely connected by one of the many types of shackles out there. The types are often found on a key chain designed to hook your keys to something else. You can know more about adjustable stainless steel D shackle from various web sources.
However, the shackle can also be found on a key chain geared towards children. People are stupid looking, cheap stuffed toys that kids love to hang their backpack is often attached to the same species.
Unlike their cousins easily opened, carabineers designed to be easily opened. They are used mainly in the climbing world. Made to withstand large amounts of weight, this chain is a staple for anyone who rides.
When a climber is preparing for the ascent, they go to every inch of their equipment with a fine-toothed comb. Their lives depend on having the best quality of rope belay, to harness, to the carabineer. There is a line that is popularly used in the climbing world when connecting shackles, "Screw the bottom so you do not mess up."