Femur
latin: Os femur
common: Thigh bone

The thigh bone (femur) is the longest and strongest bone in the human body. This bone has a length that is approximately 1/4 of the total body length. The thigh bone is almost cylindrical in form over a large part of its length.
The thigh bones connect to the hip
When we are standing upright, the femur is not really vertical, but a little bit slanted. This way the two thigh bones are close together at the knees, while being further apart at the hips, so that the thigh bones are connected to the sides of the hips.
The head of the thigh bone (latin: femoral head) is spherical in form to make it exactly fit in the socket of the hipbones. A cartilageous surface on the ball and on the socket create a smooth surface on both. Some ligaments (ligamentum teres femoris) keep the hip and the thigh bone together.
The thigh bones are not exactly
vertical when we are standing.
The lower end of the thigh bone is part of the knee joint. A shallow groove on this end of the bone is meant for the kneecap to slide over it.