The synovial joints move relatively freely. why? describe the features of a synovial joint and tell which structures support the joint to keep the bones in place.

Because there is no physical conection between the bones.

Explanation:

A synovial joint is characterized because of its articular capsule made of cartilage and other structures. Inside this there is a synovial membrane and articular cartilage. In the centre of this estructure there is a liquid that lubricates the articulation

Synovial joints, also known as diarthrosis joints are basically the heads of two bones that are not directly connected to one another, but are joined together by a series of connective tissue and a fluid-filled capsule called a synovial sac, or capsule. The particular anatomical structure of this type of joint allows articulations a range of motion that is not possible in any other types, and are most common in parts of the body that require that range of motion, like the knee joint.

Essentially, synovial joints are formed thus: two long-bone heads that come together, and both are joined by a synovial capsule. Under that capsule we have the synovial membrane which is basically the sac where the synovial fluid is kept. On the surface of each of the bone heads you will find an articular cartilage lining which connects with the synovial membranes, forming thus between them the sac where the synovial fluid is maintained.

Because the bones do not meet with one another, and because there is synovial tissue, which is much more flexible than fibrous connective tissue, these joints can move in many more plains, and angles than bones that are joined through synarthrosis and ampiarthrosis articulations.