膝

1. Origins, structure, and basic meaning

• Structure: 膝 is read as ひざ and is a noun for the knee or, when someone is seated, the lap area over the knees and thighs.
• Basic usage: Use 膝を擦りむく for injuring the knee and 膝に毛布を掛ける for placing a blanket over the lap.

2. Common pitfalls and similar expressions

• Knee and lap
  • 膝 can directly name the joint and can also cover the seated lap area in context.
• Object and location
  • In 膝を擦りむく, を marks the injured part; in 膝に毛布を掛ける, に marks where the blanket is placed.