3/8" Steel Cable holds the two leads of this oak together

Cables provide additional support to tightly forked branches or poorly attached limbs which might otherwise break, particularly in storm events. Cables are installed approximately 2/3rds the way to the top of the limb and are made of high strength steel attached with steel lags or eyebolts. Cables are somewhat ridged and provide support but can compromise the tree's ability to transfer wind energy by swaying. Cables generally last about 20 years before they are outgrown by the tree and older cables may succumb to rust and should be checked periodically, certainly whenever a tree is pruned.

The process of cabling is designed to aid in stabilizing a tree so that it can better withstand the stresses of bad weather and environmental stresses.  It is quite important to use a professional ISA approved procedure for cabling, since it is possible to weaken a tree if cables are installed improperly.  There are many different way to cable/brace a tree. Every system is designed to meet the specific need of each individual tree.