Final answer:
The carpometacarpal joints of the thumb are biaxial saddle joints, allowing movement along two planes and enabling thumb opposition.
Step-by-step explanation:
Based on the number of axes rotations, the carpometacarpal joints of the thumb (saddle) are biaxial joints. The carpometacarpal joints, specifically the first carpometacarpal joint at the base of the thumb, are examples of saddle joints. These joints are unique in that both of the articulating surfaces for the bones have a saddle shape, which is concave in one direction and convex in the other. This particular structure allows the thumb to move along two planes: it can move within the same plane as the palm of the hand, and it can also jut out perpendicular to the palm.
Saddle joints like the first carpometacarpal joint provide the ability for movement such as flexion/extension and abduction/adduction. This type of joint is what gives the thumb the capability to oppose, making it distinctive in human hand function and manipulation.