One of the key components of batting cages is the netting. Ideally, the netting should be made of a high-density polyethylene treated with some sort of UV (ultraviolet) protection, which will ensure a greater durability and will protect against damaging sunlight and weather, which could weaken the net. There are a variety of ways that the net can be knotted as well. Be sure to look for knotting that is tight and secure, so that no gaps in the net develop. For backyard batting cages, the thickness of the twine probably does not have to be as great as the pro models, but if you can find a thicker twine for your netting without too much of a difference in price, you should choose it. It ends up being more durable in the long run. Be sure any twine that stabilizes and secures the cage is also of good quality.