The operating line is reeved through the tension block from the bottom of the arbor through the rope lock. Tension blocks typically ride vertically along the arbor guide system tracks, instead of being fixed, to allow for variation in the length of the operating line. Counterweights are heavy objects that are used to balance the lineset loads in a fly system. In hemp systems, a counterweight consists of one or more sandbags, whereas counterweight systems employ metal bricks as counterweights. The term counterweight is commonly used to refer specifically to the metal counterweight bricks. Metal counterweights are lead, cast iron, or flame-cut steel. Flame cut steel bricks are most common. In any particular fly system all counterweights typically share a common, standardized footprint that matches the system's arbors, which in turn are sized to conform to line set spacing. Counterweight systems are most often designed to use either 4 or wide weights. Weights vary in thickness, typically in half-inch increments ranging from 1/2 to , with each thickness corresponding to a different mass. thick weights are most common. Counterweights are sometimes also known as bricks or simply steel. Often a rigging worker will be asked to load a number of inches of steel, which correlates to a specific mass. Weights are usually loaded from the loading bridge, but can also be loaded from the fly gallery or stage deck in some circumstances. When viewed from the top, metal counterweight is basically rectangular, typically with 45-degree angle chamfers cut at two opposing corners. A slot is cut into each end of the weight so as to enable the weight to straddle, and be laterally secured by, the arbor rods. In order to facilitate removal of weights with angle cuts, it is customary to stack the weights in alternating orientations so that the square corners of any weight will be aligned with the angled corners of adjacent weights. This simplifies removal because the square corners of each weight protrude beyond the angled corners of the weight below, serving as handles that can be easily gripped, even with gloved hands. It is customary to apply paint (typically yellow) or colored tape to the weights that counterbalance the batten (pipe) to indicate that they should not be removed from the arbor. As an additional precaution, they may be strapped in with steel strapping. When a dedicated line set carries a permanent load (e.g., main drape, orchestra cloud, etc.) the counterweight balancing the additional load may be treated in a similar fashion.