Abstract: This invention provides various configurations of spacers for use in plastic block constructions to prevent relative lateral side-to-side movement between adjacent blocks of the construction when they are connected together with interlockable tab extensions. The spacers are mounted within joint spaces between adjacent blocks of the construction and are in contacting operative engagement with the interlocked tabs to prohibit any relative lateral movement out of alignment between the blocks in the direction towards or away from the frontal or rearward planar faces of the blocks and to retain the blocks aligned in a common plane.