以外

1. Origins, structure, and basic meaning

• Structure: 以外 is read as いがい and is a noun-like expression meaning other than or except a stated reference point.
• Basic usage: Days except Sunday appear in 日曜日以外, while luggage other than a wallet appears in 財布以外の荷物.

2. Common pitfalls and similar expressions

• Role determined by the following particle
  • 以外は makes the remainder the topic, while 以外の links the excluded scope to a following noun.
• Difference from 以内
  • Unlike the expression that includes what falls within a set range, this one excludes the reference item and points to what lies outside it.