Cabling and bracing are methods of supporting old, heavy trees to ensure their strength and safety.
Cabling is the practice of placing wire rope between two branches to support both of them. Eye bolts are attached to each branch and a length of wire rope attached to the eye bolts. Often many cables are used to support pairs of branches throughout the tree.
Bracing refers to rods that are driven through branches to keep them from splitting. Crotches are often braced with rods to keep them from breaking. A hole is drilled all the way through the weak spot and a piece of threaded steel rod driven in. Large washers and nuts are screwed onto each end to hold the rod in place and secure the branch.
Both of these practices should be done only by certified arborists; good technical judgment is required in the placing of the cables and braces, and the cost of failure too steep to be risked.