The CATRig is the hierarchy that defines the CAT skeletal animation system. It is a fast, sophisticated yet flexible character rig that is designed to let you create the characters you want without having to write scripts.
Image courtesy of NHNZ
The flexibility of the CATRig character rig is the result of keeping the structure as generic as possible. With CAT's modular composition design, a spine can also be a neck. Instead of pelvises and ribcages from which limbs and other parts extend, CAT has hubs. You can add and remove different rig elements to get the exact skeleton you need for your character.
Each rig element combines geometry with special capabilities specific to its function. For example, each limb has its own, sophisticated IK system; spines have a powerful procedural spine algorithm; and hubs have a retargeting system.
Each rig also has its own layered animation system, procedural walk-cycle system, and clip/pose system. In addition, CAT includes utilities for mixing poses, resizing rigs and animation, transferring animation automatically from one rig to another, and more.