Border

Border blocks are unbreakable blocks that can only be obtained using commands.

Usage
Border blocks will not be placed if obtained in inventory. They can only be placed using. When placed, all blocks above and below the border block will be filled with invisible blocks that cannot be broken, and no blocks can be placed on top and bottom of the border block. Border blocks cannot be broken, even in creative mode, they can only be broken using commands. When you break a border block, all invisible borders on top and bottom of the block will break and then you will be able to place blocks.

Trivia
Border block and Cobblestone Wall have same block models and inventory models.