上

1. Origins, structure, and basic meaning

• Structure: 上(うえ) denotes a position higher than a reference object or place, or its top surface.
• Basic usage: It describes a partly read book lying on a desk.
  • 机の上に読みかけの本が置いてある。

2. Common pitfalls and similar expressions

• On top of and above
  • 机の上 can refer to contact with a desktop, while 頭の上 can refer to space higher than the head. Whether English uses on or above depends on contact and context.