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The Story of Soundex

Originally developed by Margaret K. Odell and Robert C. Russel at the U.S. Bureau of Archives to simplify census taking in the early 1900s, the Soundex algorithm isn’t perfect but works quite well to overcome the problem of alternative spellings of Names.

The Soundex Algorithm makes the following assumptions about words in English:

  • Vowels contribute less to the sound of the word than consonants do and may there fore be disregarded unless they occur at the beginning of the word.
  • The Letters H, W, and Y make only minor contributions to the sounds of most words and therefore can also be disregarded again, unless they occur in the beginning of the word.
  • Similar sounding consonants that appear together in a word sound a lot like single consonants, and for analytical purposes can be reduced to a single consonant sound.
  • Soundex reduces any word into a four digit code that quantifies how that word is pronounced. If the codes for the two words match, then the words sound roughly the same.

The SOUNDEX system provided for variant spelling or misspelling of a name. Phonetic Compression Schemes such as SOUNDEX cannot generally provide for random errors such as typing an incorrect character, or transpositions (where the order of the letters is reversed).


Soundex is a phonetic algorithm for indexing names by sound, as pronounced in English. The goal is for homophones to be encoded to the same representation so that they can be matched despite minor differences in spelling.[1] The algorithm mainly encodes consonants; a vowel will not be encoded unless it is the first letter. Soundex is the most widely known of all phonetic algorithms, as it is a standard feature of MS SQL and Oracle, and is often used (incorrectly) as a synonym for "phonetic algorithm". Improvements to Soundex are the basis for many modern phonetic algorithms.


Soundex was developed by Robert C. Russell and Margaret K. Odell and patented in 1918[2] and 1922.[3] A variation called American Soundex was used in the 1930s for a retrospective analysis of the US censuses from 1890 through 1920. The Soundex code came to prominence in the 1960s when it was the subject of several articles in the Communications and Journal of the Association for Computing Machinery, and especially when described in Donald Knuth's The Art of Computer Programming.[4]

The National Archives and Records Administration (NARA) maintains the current rule set for the official implementation of Soundex used by the U.S. Government.[1] These encoding rules are available from NARA, upon request, in the form of General Information Leaflet 55, "Using the Census Soundex".

Soundex variants

A similar algorithm called "Reverse Soundex" prefixes the last letter of the name instead of the first.

The NYSIIS algorithm was introduced by the New York State Identification and Intelligence System in 1970 as an improvement to the Soundex algorithm. NYSIIS handles some multi-character n-grams and maintains relative vowel positioning, whereas Soundex does not.

Daitch–Mokotoff Soundex (D–M Soundex) was developed in 1985 by genealogist Gary Mokotoff and later improved by genealogist Randy Daitch because of problems they encountered while trying to apply the Russell Soundex to Jews with Germanic or Slavic surnames (such as Moskowitz vs. Moskovitz or Levine vs. Lewin). D–M Soundex is sometimes referred to as "Jewish Soundex" or "Eastern European Soundex",[5] although the authors discourage the use of these nicknames. The D–M Soundex algorithm can return as many as 32 individual phonetic encodings for a single name. Results of D-M Soundex are returned in an all-numeric format between 100000 and 999999. This algorithm is much more complex than Russell Soundex.

As a response to deficiencies in the Soundex algorithm, Lawrence Philips developed the Metaphone algorithm in 1990 for the same purpose. Philips developed an improvement to Metaphone in 2000, which he called Double Metaphone. Double Metaphone includes a much larger encoding rule set than its predecessor, handles a subset of non-Latin characters, and returns a primary and a secondary encoding to account for different pronunciations of a single word in English.

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