音読み: イチ
訓読み: none
画数: 7
部首: 士（さむらい）
Meaning
One (formal) / First / Single.
- 壱万円 — ten-thousand yen (written in formal numerals)
- 壱岐 — Iki (island name)
- 壱号 — Number One; Unit One
壱 is the formal (banking/legal) way to write “one,” used on checks, receipts, and official documents to prevent tampering. It derives from the older character 壹 and is distinct from the everyday numeral 一. When you need a figure that can’t be altered easily, you switch to these formal numerals (大字) — 壱, 弐, 参, etc.
Think of old-style numbers written in formal documents — to avoid fraud, they used special complex numerals called 大字 (だいじ). 壱 is one of them, used instead of 一. You’ll often see it in 壱万円, sometimes paired with the old form 萬. Other digits also had complex forms — 拾 for 十, 佰 for 百, and 阡 for 千 — but 壱 is by far the one most commonly used today.
壱岐 — Iki (island in Nagasaki Prefecture)