The knee is a complex, compound, condyloid variety of a synovial joint. It actually comprises two separate joints. The femoro-patellar joint consists of the patella, or "kneecap", a so-called "sesamoid bone" which sits within the tendon of the anterior thigh muscles, and the patellar groove on the front of the femur through which it slides. The femoro-tibial joint links the femur, or thigh bone, with the tibia, the main bone of the (lower) leg. The joint is bathed in a viscous fluid which is contained inside the "synovial" membrane, or "joint capsule". The recess behind the knee is called the "popliteal fossa".