As the knee is bent, the two rounded ends (condyles) of the femur (thighbone) are exposed.
The notch between the condyles is also revealed (intercondylar notch) as well as the cruciate ligaments which tether femur and tibia together. When the knee is straight, the condyles roll over and these structures are concealed.
This is the reason for the knee surgeon to do many of the surgical procedures with the knee bent. Normally the patella (kneecap) and its tendon is in the way, and the keyhole surgeon creates the portals for the instruments to the side of the patellar structures. For the purposes of this first illustration, the patella and its tendon have been removed to allow you to see into the knee.
Inside the intercondylar notch are the two cruciate ligaments, which appear to cross over one another. The one in the front is called the anterior cruciate ligament and the one behind is called the posterior cruciate ligament.
The two crescentic shock absorbers - the menisci - are situated between the femur and the tibia. They are seated on the flattened top of the tibia (tibial plateau). Although it is not shown here, they have ligamentous attachments to both bones via the capsule - the waterproof casing of the knee. The medial meniscus is the one most commonly injured because it is less mobile that the lateral meniscus, but injury to the lateral meniscus may be especially problematic as its anatomy is more complex.
The patella normally glides in the groove (trochlear groove) between the two condyles as the knee bends and straightens. It is tethered below to the tibia bone via its tendon (patellar tendon). Above, and not shown here, the patella forms part of the quads muscle group (quadriceps) and is attached to the muscle via the quadriceps tendon.
This very simplified way of illustrating the joint by stripping away details is merely to make the primary relationships clear, and more sophistication will be added to the drawings as you go through this anatomy section.