from a point which is one quarter the longitudinal length of the block from its end and that the centerlines are spaced apart a distance equal to the separation of the centers of the pin 30 and through hole 32 on the straps 20 the other two straps 20 are similarly positioned from the opposite end of the block the straps 20 traversing the bottom of the block 10 exists at positions mirroring the top straps 20 and interface with underlying blocks in an identical way fig2 and 3 illustrate the way in which the blocks 10 can be stacked to form a concrete structure having the pins 30 and holes 32 of the stacked blocks 10 interfacing as just described as can be seen in fig2 each overlying block 10 overlaps one half of the underlying block 10 those blocks 10 which are placed at a 90 degree angle to the adjacent blocks 10 above and below at the corners of the concrete structure similarly overlap by one half of their lengths fig3 shows how the pins 30 and holes 32 of the straps 20 of the stacked blocks 10 interface the pins 30 on the straps 20 of the underlying block.