Abstract: A block and various block assemblies are disclosed. The block has at least one side including one or more tapered slots. Each of the slots has one or more apertures. The at least one side has a bottom end comprising at least one footing and a top end comprising a groove. Blocks are stacked by fitting the footing of one block into the groove of another block. A block assembly comprises a first block, a second block, a double key, and connector tubes. The first block is connected to the second block using the double key, which is inserted into a slot on each of the blocks. A connector tube is threaded through apertures on the slots and the double key.