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LingakuLingaku万が一 + [sentence (conditional: たら/ば/としたら/の場合)]
万が一の + こと/場合/時 + (に/には/も)
万が一 + にも
万が一のために
Note: 万が一 functions as an adverbial expression or noun; often placed at the beginning of a clause to introduce an unlikely hypothetical.
万が一 (まんがいち) literally means "one in ten thousand" and is used to express the idea of an extremely unlikely event – "just in case," "in the unlikely event that," or "in the worst-case scenario." It emphasizes that the speaker considers the possibility very remote but still worth preparing for or considering.
万が一 is most common at the beginning of a conditional clause, often paired with たら, ば, のばあい場合, or としたら to set up the hypothetical scenario.
When used as a noun modifier with の, 万が一 forms phrases like 万が一のこと（the worst-case scenario）, 万が一の場合（in the unlikely event）, and 万が一の時（if the worst happens）.
万が一 carries a stronger sense of improbability than もしも or もし. While もし simply means "if," 万が一 highlights that the event is extremely unlikely but not impossible. It is used in serious contexts such as emergencies, accidents, health, and important decisions. It can appear in both formal and informal speech but is particularly common in written language, announcements, and advice-giving contexts.