The Soundex is a system of assigning an Alpha-Numerical code to each surname. It codes names by the way they sound, rather than the way they are spelled. Why do you need to know the Soundex value or your surnames? Because there are Soundex indexes to the 1920, 1910 (some), 1900 and 1880 (some) census.
The Soundex consists of a letter (always the first letter of the surname) followed by three numbers. Numbers are assigned by the following guide, because these letters groups share a similar sound, or the sound is created in the same place in the mouth:
1 B P V F
2 C S G J K Q X Z
3 D T
5 M N
To code your surname:
Disregard A, E, I, O, U, W, Y and H
In short names, if there are less than 3 numbers, use zeroes.
Double letters count as a single one.
On long names, once you have 3 numbers disregard the rest of the letters.
For example, PHILLIPS is coded P412: P because it is the first letter of the surname, 4 because H and I are disregarded, so the first letter to be coded is L. The second L is disregarded, as is the I. The next letter to be coded is P (1) and then S (2).
Kennedy is K530
Davidson is D132
McMurphy is M256
Now try a few on your own.
Visit GeneaologyInTime Magazine for a more comprehensive definition of Soundex