The cable is never piano-string tight, there will always be a natural amount of sag in the cable.  Sag is the amount the belly of the cable dips below your end anchor attachment point on the tree.  This is important for two reasons: you never want your riders to bottom out, nor do you want the cable to be over tensioned or too tight.Instructions for how to measure sag can be found in the Zip Line Gear handbook included with your purchase. You can also view a digital version of the “Cable Slope & Sag” section of the handbook right here.Cable Tension is measured by Cable Sag:The cable, when bearing a test weight, should sag below the end where the cable is attached.The sag needs to be approximately 2% of the Zip Line’s total length. (2 feet per 100 feet of cable)The sag is measured at the cables lowest point. !! Warning!! Do not attempt to ride your zip line without first testing.