Fence

Fences are decorative wooden wall blocks added in the Secret Friday update 8. They count as one and a half blocks tall for player collision, and one block tall for all other purposes. So they cannot be jumped over from the same level as the fence but can have other blocks placed on top of them. They occupy the centre of blocks and automatically connect to one another. Fences can be placed on top of both kinds of pressure plates and they cannot be placed on top of one another directly, but can be stacked through other means. Standing on top of a fence in multiplayer counts as an illegal move, and can cause players to get stuck.

Stacking
You can stack multiple fences on top of each other by creating a wall of blocks one layer shorter than the intended height of the fence and working down. Removing blocks from underneath the fence doesn't cause it to collapse like cacti and doors, so the lower layers of the wall can one-by-one be replaced by fences easily.