The joint itself. Used in medical, everyday, and figurative contexts.
転んで膝をすりむいた。
I fell and scraped my knee.
膝が痛くて正座できない。
My knees hurt so I can't sit in seiza.
lap; knee and thigh (while sitting)
Refers to the area from the knees to the thighs when sitting, especially as a surface for holding something or someone.
猫が膝の上で寝ている。
The cat is sleeping on my lap.
子どもを膝に乗せて本を読んだ。
I read a book with the child on my lap.
It hurts so much that I can't bend my knee.
Lately my knee hurts when I go up stairs.
My father sat my little brother on his lap and read him a picture book.
I fell and scraped my knee.
He sat down in the chair and crossed his legs.
The child is sitting on his mother's lap being affectionate.
one knee
both knees