A modular building block comprises a first load-bearing surface having a key. There is a recess in the key. The recess has a shoulder disposed therein and side walls extending from the shoulder to a mouth thereof. A reinforcement rod extends into the recess and a coupling for connecting vertically aligned reinforcement rods is disposed in the recess. The reinforcement rod threadedly engages the coupling. A locking plate engages the coupling to prevent the coupling from rotating relative to the locking plate. The locking plate is shaped to sit on the shoulder so that rotation of the locking plate within the recess is prevented by the side walls of the recess.