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Background is where walls can be placed and collected with hammers. In addition to blocks, furniture is also placed in the background. The only background block(s) which can't be destroyed with a hammer are Trees. Trees can be chopped down with an axe.
Walls are tiles placed in the Background. All walls prevent Monsters from spawning, allow many furniture items to be hung on them, and are required for NPC housing. Walls do not prevent players or monsters from passing through. There are several different types of walls. With the exception of Dirt Wall, Ice Wall, Gem Wall, and Dungeon Walls there is no difference between them other than their appearance. Walls can be placed in the Underground layers once stones start to appear in the background. Most naturally-occurring Walls can only be broken from an edge, using either a hammer-type tool or an explosion (Bomb or Dynamite), and do not drop anything when broken. Other Walls can be broken from any point and will drop a wall place-able block. Some walls can be crafted back into bricks.
One brick will craft 4 wall blocks of that type. Walls can only be placed next to bricks or other Walls. Placed wall blocks can only be broken with a Hammer. Naturally occurring dirt walls generally need to be destroyed from the outside in (i.e., you cannot knock a hole in the naturally occurring dirt wall deep underground). To calculate how many blocks are needed to make enough Wall to cover an area, multiply the number of blocks long by the number of blocks tall then divide by four.
Types of Walls
These walls can only be obtained though the destruction of the dungeon background with a hammer, hamaxe, or explosives. Explosives will only destroy the walls that aren't blocked by the actual bricks, because dungeon bricks cannot be destroyed with Bombs or Dynamite: Blue Brick Wall, Green Brick Wall, Pink Brick Wall.
Before the 1.2 update, Ebonstone and Hellstone walls existed in-game, but only in naturally generated Corruption chasms and Hell Shrines respectively. The item, however, did not drop, which made them unobtainable. Another type of unobtainable Mud Background Walls were added in the 1.0.6 patch, and are generated within a 1.0.6 and above world in the Jungle. You however could not craft them, even though the change log said you were, in fact, able to. However, they can be created as of the current update.Breaking mud background walls with a Hammer/Hamaxe will turn them into "dust" much like Dirt walls, Hellstone walls, etc.