What type of joint allows for flexion and extension by fitting a convex projection into a concave depression?

The hinge joint is specifically designed to permit flexion and extension movements, functioning much like a door. This type of joint features a convex surface of one bone that fits securely into a concave surface of another bone, allowing movement primarily in one plane.

For example, the elbow and knee joints are classic instances of hinge joints. When you bend your elbow or knee, the convexity of the bone fits smoothly into the concavity of the adjacent bone, facilitating the bending motion (flexion) as well as the straightening motion (extension).

Other joint types mentioned, such as the ball and socket joint, allow for a greater range of motion including rotation, while pivot joints allow for rotation around a single axis. Gliding joints, on the other hand, enable limited sliding or gliding movements without the elbow-like bending structure of hinge joints. Thus, the hinge joint is uniquely suited for the specific movements of flexion and extension due to its convex-concave interlocking structure.