Slab

Slabs are blocks that allow the player to change elevation without jumping. They are not as compact as stairs, requiring twice the horizontal space for an equivalent change in elevation. Stone slabs were introduced to the game on October 24, 2009, in Survival Test 0.27, whereas sandstone, wooden, and cobblestone slabs were added in the Beta 1.3 update on February 22, 2011.

All four types of slab have the same data values; like wood and colored wool, they are differentiated by their damage values: stone slabs have a damage value of 0, sandstone slabs are 1, wooden slabs are 2, and cobblestone slabs are 3.

Stone Slab
The only slab that - with 2 slabs in the same place - don't look like the normal block is the stone slab. Why? Because it has a line in the half of it.

Crafting
Slabs can be made from three wooden planks or three stone, cobblestone, or sandstone blocks, producing three slabs per crafting operation.

Behavior
Like other partial blocks, slabs are treated as a whole block with other blocks, such as dirt, stone, and glass, and liquids. Two slabs of the same type (e.g. two stone slabs) can be placed one on top of the other to make a single full-size block, but different slab types cannot be mixed in this way.

Sandstone, wooden, and cobblestone double-slabs look exactly the same as their full block counter-parts, but take a longer time to break and drop two slabs.

Due to a bug, wooden slabs can currently only be collected with a pickaxe, and upon breaking, a wooden slab produces the stone slab's particle effect. Wooden slabs also are not affected by fire and have a stronger blast resistance than wooden planks, making them a useful building material but at a 2:1 ratio. This is likely due to the fact that the slab types are differentiated by their damage values instead of by different data values, similar to wood, coal or charcoal, and colored wool.

History
Stone slabs were first introduced in Classic mode. In Classic and Survival Test, stone slabs could be obtained by mining a coal block because of the lack of inventory and crafting.

Stone slab blocks were called stair blocks before the current stairs were added; after this, they were known as steps (the two forms being single steps and double steps) before all items got official names upon the Beta release.

Before the additional slabs were added, a double Stone Slab would only yield one slab when broken. Since the Beta 1.3 update, all double slabs yield 2 of their respective single slabs when broken. Destroying double slabs with TNT, however, still only yields single slabs (when the slabs aren't simply destroyed by the explosion).

Before Beta 1.3 came out, stone slabs were made with cobblestone instead of stone, but that update introduced cobblestone slabs to the game and changed the recipes for Pressure Plates and stone slabs so that there wouldn't be any conflicting recipes.

Trivia

 * Despite the fact that sneaking gives a player an apparent height of 1.5 blocks, doing so does not allow the player to walk over a single slab with one block of air above it.
 * A player can not walk from a block of Soul Sand to a slab without jumping.
 * Wooden slabs are not flammable and have to be destroyed by a pick to drop something, possibly due to the fact that they share the same data value as other slabs. This can be used in SMP to help prevent griefing.
 * Single slabs have the tendency to let through arrows shot from above.
 * Single slabs will destroy gravel and sand blocks that fall onto them, the same as torches.
 * Being pushed onto a slab by water will force the player to jump.
 * A single slab cannot be made into a double slab if there is another solid block directly above the single slab.
 * Single slabs are treated as transparent by the game. Because of this, they do not cause suffocation, you cannot place torches or other fixtures on them, and chests with single slabs above them can still be opened.
 * The exception to this is that when two slabs are placed side-by-side in water, a ladder can be placed on them. However, any ladder placed in this way is invisible.
 * Since the Beta light update, slabs now let a small amount of light pass through their edges. This light is only visible with Smooth Lighting turned on, and does not affect mob spawning or other light-dependent processes.
 * When a slab is placed on top of ice, the slab has the same "slippery" contents as the ice below it.
 * If a stack of slabs of the same type is placed from the top down (leaving an empty half-block high space between each pair of slabs), and another of the same type of slab is placed on or one block above the topmost slab, all the slabs will collapse into a single double slab where the bottommost slab was.
 * Similarly, it is not possible to place one slab above another of the same type leaving a half-block of empty space between them. The second slab always gets placed directly on top of the first, making a double slab.
 * If half-blocks are placed below TNT, it will significly reduce their damage. By making a floor of slabs, the TNT will lower its explosive power to two blocks instead of six.

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