The present invention relates to a rectangular construction block having on the longitudinal axis of two opposed surfaces symmetrically arranged inter-engaging means in the form of protrusions and/ or depressions symmetrically spaced from a central bore extending between the two surfaces and having end surfaces with vertical grooves, such that when a pair of blocks are placed together end to end the two form a bore identical to the central bore and when a further block is placed on the pair of blocks the protrusions on one block engage in the depressions of the pair of blocks and vice versa.