万一

1. Origins, structure, and basic meaning

• Structure: 万一 is read まんいち and refers to an unlikely but possible contingency.
• Basic usage: Use it when preparing for an emergency or presenting a low-probability condition, as in 万一に備える and 万一…たら.
  • 万一に備えて、予備の鍵を家族に預けました。 describes leaving a spare key as a precaution; 万一電車が止まったら、この番号に連絡してください。 gives instructions in case the train stops.

2. Common pitfalls and similar expressions

• 万一に備えて
  • に marks the contingency being prepared for, and the following clause states the advance precaution.
• 万一…たら
  • With たら, 万一 presents an unlikely situation as a condition. It does not predict that the event will occur.
• 万一 and もし
  • もし introduces conditions broadly, while 万一 more strongly suggests preparing for a rare or undesirable event.