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Soundex definition

Soundex is a phonetic algorithm that indexes words based on their sounds. It was developed to help match names despite minor differences in spelling. Soundex converts a word into a code that captures its primary consonant sounds.

How Soundex works

Soundex converts a word or name into a code consisting of a letter followed by three numbers. The algorithm processes each word based on predefined rules:

  1. 1. Retain the first letter of the word.
  2. 2.Remove all occurrences of 'h' and 'w' except the first letter.
  3. 3.Replace letters with digits according to the Soundex coding rules:
    • A, E, I, O, U, Y, H, W are ignored (except the first letter).
    • B, F, P, V = 1
    • C, G, J, K, Q, S, X, Z = 2
    • D, T = 3
    • L = 4
    • M, N = 5
    • R = 6
  4. 4.If two or more letters with the same number are adjacent in the original name (before step 1), remove all but the first.
  5. 5.Return the first four characters (letter and three digits). If necessary, pad with zeros to ensure four characters.

For example, the surnames "Smith" and "Smyth" both encode to S530. "Johnson" and "Johnsen" both encode to J525.


Robert C. Russell and Margaret King Odell patented Soundex in 1918 and 1922. The U.S. government adopted it for the 1930 Census to assist with indexing surnames. Its ability to handle varied spellings made it particularly useful for genealogical research, where records often contain misspellings or alternative spellings of surnames.