The Soundex system is the means established by the National Archives to index the US censuses (beginning with 1880). It codes together surnames of the same and similar sounds but of variant spellings. Soundexes are arranged by state, Soundex code of the surname and given name.
Soundex codes begin with the first letter of the surname followed by a three-digit code that represents the (first three) remaining consonants.
Soundex Coding guide:
The letters AEIOUYH and W are not coded.
Example: Camp would be C510
Note that surname prefixes such as van, Von, Di, de, le, d' dela or du are sometimes disregarded in alphabetizing and in coding.