These are recommended for use with all expansion joints (especially if piping system is not properly anchored) to prevent damage from excessive motion. The 'control' and 'limit' allows the specified pipe expansion and contraction (axial compression and extension). They are sold per set (set includes 2 rods, 4 plates, 8 nuts/washers). For setting up a limit rod only, it uses less nuts/washers, but utilizes all the same parts. The number of sets required per expansion joint increases with the engineered pipe size and surge/test pressures of the system - refer to mfg recommendations/table. Minimum requirement is 1 set with rods setup on opposing sides.