Final answer:
A saddle joint is a type of synovial joint that allows movement in multiple planes. The first carpometacarpal joint is an example of a saddle joint that enables the thumb to move in different directions. The acromioclavicular joint is not a saddle joint.
Step-by-step explanation:
A saddle joint is a type of synovial joint in which the articulating ends of both bones have a saddle shape. The primary example of a saddle joint is the first carpometacarpal joint between the trapezium and the first metacarpal bone at the base of the thumb. This joint allows the thumb to move in multiple planes, giving humans their distinctive 'opposable' thumbs. The acromioclavicular joint is not a saddle joint.