B1
the middle joint of the leg that allows the leg to bend:
The baby was crawling around on its hands and knees.
He got/went down on his knees (= got into a position where his knees were on the ground) in front of the altar.
She took the child and sat it on her knee (= on the part of the leg above the knee when sitting down).
the part of a piece of clothing that covers the knee:
She was wearing an old pair of trousers with rips at the knees.