On the inner aspect of the knee the medial collateral ligament (MCL) connects femur to tibia, and on the outer aspect the lateral collateral ligament (LCL) connects the femur to the fibula, the thinner bone adjacent to the tibia.

Image of knee showing the position of the collateral ligaments

Both sets of ligaments - collaterals and cruciates - are necessary to maintain full stability of the knee joint.

For clarification, a ligament is a fibrous structure which connects from bone to bone (in contrast to a tendon which connects from bone to muscle).