# Why is a fullwidth digit not an ASCII digit?

U+FF11 has a compatibility decomposition to U+0031. NFKC folds it to the ASCII digit. NFC leaves U+FF11 in place. Code points: U+FF11. Compatibility equivalence is not canonical equivalence. NFKC also folds other compatibility characters, so it is not a general cleaner. Destination acceptance was not tested. A non-equivalent fallback is "1" (ASCII digit one). Drops the fullwidth form. Running NFKC on a longer string can change more than this digit. Exact string match: no. NFC match: no. Appearance and meaning were not tested. Font coverage, shaping, and application acceptance were not tested, so those results are unknown.

## Code points

- U+FF11 FULLWIDTH DIGIT ONE

## What was checked

### Fullwidth digit and an ASCII digit

U+FF11 has a compatibility decomposition to U+0031. NFKC folds it to the ASCII digit. NFC leaves U+FF11 in place.

Compatibility equivalence is not canonical equivalence. NFKC also folds other compatibility characters, so it is not a general cleaner. Destination acceptance was not tested.

Use U+0031 when an ASCII digit is required. Keep U+FF11 when the fullwidth form is the text.

## What this does not prove

No OS, font, browser, application, field policy or rendering engine was tested. Unicode validity and segmentation do not establish actual display or acceptance.

Unicode release recorded with this answer: 18.0.0.
Machine-readable JSON: https://copypastecharacters.com/reference/v1/answers/why-a-fullwidth-digit-is-not-an-ascii-digit.json
Exact lookup: https://copypastecharacters.com/reference/v1/exact/FF11.json
HTML: https://copypastecharacters.com/answers/why-a-fullwidth-digit-is-not-an-ascii-digit/

Do not invent a platform-support claim from this data.