Slabs are half-blocks which, unlike stairs, do not stop the player from sprinting, which makes vertical elevation equally quick and horizontal speed twice as fast as stairs while sprinting on slabs. Note: from 13w05a this is no longer the case - stairs do not prevent sprinting. Upside-down slabs occupy the top half of their block space rather than the bottom half. To place a slab upside-down, right-click on the bottom of a "ceiling" block, which can be removed after the slab has been placed, or by right clicking the top half of another block, if the player wishes to place it on the side of another double or single slab block.
Solid BlockNoneNoNo (Double Slab) Yes (Single Slab)No30 (Stone), 15 (Wood)Stone: YesSandstone: NoWooden: YesCobblestone: YesBricks: NoStone Brick: YesNether Brick: NoQuartz: NoYes (64), same type only?Dirt, Stone Survival Test 0.26 Sandstone, Wooden, Cobblestone Beta 1.3 Bricks, Stone Brick, Smooth Stone Full Half-Slab Beta 1.8 Upside-down Slabs 1.2 Preview Wooden Slabs (Oak, Birch, Spruce, Jungle) 12w17a Nether Brick Slabs 12w49a Quartz 13w02a Smooth Sandstone Full Half-slab 13w04aDouble Stone Slab dec: 43 hex: 2B Single Stone Slab dec: 44 hex: 2C Double Wooden Slab dec: 125 hex: 7D Single Wooden Slab dec: 126 hex: 7E