A slab is half a block. If you say "half-slab," you're literally saying "half half block," or in other words, "quarter-block." A slab is not a quarter of a block. It is a half of a block. There is no such thing as a quarter block.A slab is half a block. If you say "half-slab," you're literally saying "half half block," or in other words, "quarter-block." A slab is not a quarter of a block. It is a half of a block. There is no such thing as a quarter block.