Abstract: A modular system includes a plurality of building units, each having at least one countersunk aperture for alignment with a countersunk aperture of another building unit; and fastening devices for securing the building units together in any of a plurality of configurations to form the modular system, each fastening device including a first bolt for engagement within a first countersunk aperture such that the upper surface of the first bolt is substantially flush with the inner surface of the wall of the building unit containing the respective countersunk aperture, a second bolt for engagement within a second countersunk aperture that is in alignment with the first countersunk aperture such that the upper surface of the second bolt is substantially flush with the inner surface of the wall of the building unit containing the respective countersunk aperture, and a nut positioned within the first and second countersunk apertures for securing the first and second bolts together.