The "lost rule" of Soundex.
Q. What's the “lost rule” of Soundex, and how does it affect the codes for my ancestral surnames?
A. Soundex is an indexing system that groups surnames together by codes based on their consonant sounds. Each code begins with the first letter of the name, followed by three numbers, representing consonants in the name (see the Soundex key). To form the three-digit code, first strike out vowels and y. Most of us were taught to strike out h and w and treat them as vowels — but some names need the “lost rule” or the “H and W rule.”