予備

1. Origins, structure, and basic meaning

• Structure: 予備, read よび, combines in advance 予 with preparing 備 to denote a reserve or spare.
• Basic usage: It modifies something prepared in advance as a replacement when needed, as in 予備の電池 or 予備の鍵.

2. Common pitfalls and similar expressions

• Grammar and combinations
  • It means a spare kept apart from the primary item, not merely an additional item already in use.