Minecraft wiki entry for Chain, associated text: Chains can also generate in mineshafts. They generate on the sides of a wooden bridge (a mid-air corridor) when the distance between the bridge and the highest solid block below it is higher than the distance to the lowest solid block above it. Chains here generate vertically in a pillar between the bridge and the ceiling. The lowest block of the pillar, connecting the chain to the bridge, is always an oak fence.