Minecraft wiki entry for Chain, associated text: Chain is made out of two iron nuggets and one iron ingot, but its blast resistance is the same as the block of iron.
A vertical chain, being a solid, but not a full block, allows for many sizes of mobs and players to pass alongside each piece horizontally.
Despite its name, it cannot be crafted into chainmail armor.[3]
If a sticky piston pulls a chain that is more than 1 block long, the chain breaks.