What is one and how do they work?

Go to http://www.kellerbowsights.com/ and look there site over and it should ansewer some of your ?'s. Other companies that make pendulums off the top of my head is Impact archery, Savage, Scout mountian, pse and the list goes on. I think the price range for pendulum sites are depending on manufactor $20.00-$90.00. I personally wouldn't use anything else for one reason I hunt only from treestands and one thing to consider with a pendulum is you need to hunt pretty flat ground like the top of the hill because if you shot down a drop from the levelof ground your stands on it will be off because of the extra distance.