Ballchain is that stuff you see at banks keep pens from being stolen, and in id badge necklaces (or any other place you want an adjustable length loop). Apparently it comes in a variety of sizes (diameter of the balls), from the aforemention bankpen size up to 1/4" or more.
What the heck are the larger sizes used for? I didn’t even know they came that big until they were used on an episode of Mythbusters.
We use the (approx.) 1/4" size for leashes and “jesses” for birds of prey. Traditionally, falconry gear is made of leather. Today other materials including twine/rope of various natural or artificial fibers may be employed. Some few individual birds at our facility chew through and destroy leather jesses and leashes almost as fast as we can create new ones. For these, we have experimented and achieved success using bead chain. It is extremely flexible, and since each bead is its own swivel, it is very resistant to problems caused by “twist”, which can be an issue with stiff leather, especially with smaller raptors.