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1. Origins, structure, and basic meaning

• Structure: 旧 has 5 strokes; 旧友 is read きゅうゆう, and 旧姓 is read きゅうせい.
• Basic usage: 旧友 is a friend known from earlier times; 旧姓 is a surname used previously.

2. Common pitfalls and similar expressions

• The history of a friendship and a former name
  • 旧友と再会した means meeting an old friend again. The word 旧友 concerns the history of the friendship, rather than necessarily the friend's age.
  • 引き続き旧姓を使っている means continuing to use a former surname. The label 旧姓 alone does not establish that the name is no longer used in any present context.