A choker chain usually doesn't look any different from a regular chain, but is often made of grade 100 chain, which is higher strength chain than the normal stuff. Also, all the true choker chain I've ever seen was 3/8", though this may only be coincidence. I wouldn't want to use anything smaller, but I expect the big crawlers use 1/2" chain.
The main visual difference with a true choker chain is the hook on the end. It's a slip hook with a much narrower "throat" to make it more difficult for the chain to slip out of the hook when you're using it.
There's also a "Choker chain" thread I started in the Attachments Discussion that talks about a particular kind of "square link" choker chain available from Labonville, a supplier of logging equipment. It's incredible chain, as I have found out from experience. It's much less prone to slipping than standard chain. Also see the thread "Grubbing tongs" that I started in the Attachments Discussion, though this only applies to pulling up unwanted trees and brush.
Typically, the choker chain is attached to as much "pulling" chain (or cable) as is necessary via a grab hook. The pulling chain is connected to the tractor's drawbar via a drawbar-mounted grab hook or shackle. If you use a shackle, you'll need a grab hook on that end of the chain, too, so you can pass the chain through the shackle and hook it to itself. Or, you can do what I did, which is get a grab link, an oblong ring (sort of keyhole shaped) with a narrow throat at one end that the chain hooks in. I put the grab link on the drawbar-mounted shackle, so I can use either.
FWIW, as you'll see in the "Choker chain" thread I referenced above, some folks much prefer choker cables instead of chains.
You can find a picture of a shackle at
http://www.slingchoker.com/sling2/cm/vi/vi4.htm. If you go to
http://www.labonville.com/product3.html#cable, the leftmost chain wrapped around the log has a grab ring at the top and a choker hook lying on the log.
I hope this helps. I'm sure some others will jump in, too.
Mark
