Final answer:
Convex surface of one bone fits into the concave surface of another -> allows movement in one direction. Hing* joint.
Step-by-step explanation:
A hing* joint involves the fitting of a convex surface of one bone into the concave surface of another allowing movement in a single direction, exemplified by the joints at the elbows and phalanges which enable flexion and extension.
When discussing a convex surface of one bone fitting into the concave surface of another, which allows movement in one direction, we are referring to a hing* joint. In a hing* joint, the slightly rounded end of one bone fits into the slightly hollow end of the other bone, allowing for flexion and extension only. An example of a hing* joint is the elbow, where the articulation between the trochlea of the humerus and the trochlear notch of the ulna occurs. The same type of joint is found in the phalanges, allowing the fingers to bend and straighten.