Abstract: A modular post block may include a core through the block body and three interlocking portions disposed on the inner perimeter surface of each block that project outward from one of the bottom surface or the top surface of each block. First and second interlocking portions are disposed on one surface of the inner perimeter and third interlocking portion is disposed on the opposite side. Blocks can be stacked in a column by horizontally rotating each successive block 180 degrees. Interlocking portions of a first block then project into the core of a second block and the third interlocking portion of the first block fits at least partially between two interlocking portions of the adjacent second block.