NFD

NFD, or Normalization Form D, is a Unicode normalization form defined as canonical decomposition. It transforms characters into a fully decomposed sequence consisting of a base character and one or more combining characters. The result preserves the text’s semantic content while potentially increasing its length, and is used to enable consistent processing of visually equivalent strings.

In NFD, each character is replaced by its canonical decomposition, and the resulting sequence is then reordered so that combining marks appear in a canonical order. For example, the character É (U+00C9) decomposes to E (U+0045) followed by the combining acute accent (U+0301). Some characters decompose into multiple code points, and combining marks can appear in various positions relative to the base character until canonical ordering is applied.

NFD is distinct from NFKD, which applies compatibility decomposition in addition to canonical decomposition. Compatibility decomposition can alter formatting or meaning for certain characters, whereas NFD preserves canonical equivalence without such compatibility mappings. After decomposition, normalization procedures may also recompose or reorder components as needed by downstream tasks.

Common uses of NFD include canonical equivalence checks, text search and indexing, and other text-processing tasks where a stable, decomposition-based representation improves reliability across different input forms. It is one of several normalization forms defined in the Unicode Standard, often used in pipelines alongside NFC, NFKC, and NFKD to achieve the appropriate balance between canonical equivalence and storage efficiency.

See also: Unicode normalization, Unicode Standard Annex #15, NFC, NFKC, NFKD.

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