I first came across this post about hidden clues in the GRO indexes quite a while ago, but have only recently tried out the method described. I won't repeat the instructions here, as the original post is quite detailed. In essence, the indexes are organised by year, quarter and registration district. Within any registration district, entries are ordered by sub-registration district for births and deaths, or parish for marriages. If all entries for any given year/quarter/registration district of the indexes are sorted by page, it may be possible to infer the sub-registration district or parish for any given entry. This can be useful when trying to choose which certificate to order when researching a common name, or where to begin searching parish records.

Birth and Death indexes are ordered by sub-registration district number. The numbers for the sub-districts can be found in the Registrar General's annual reports.

Marriage indexes are ordered by parish alphabetically, followed by Roman Catholic, Register Office, Quaker, Non-conformist and Jewish marriages. The parishes within any registration district can be found on the Vision of Britain website. First, find the registration district (generally has RegD/PLU at the end of the name). Then look at 'Lower level units'. The parishes are of type 'Parish-level Unit'.

• Vision of Britain
• FreeBMD Find Page Entries, a quick way of finding how many entries are on each page for any given quarter
• Online Historical Population Reports is a useful website, but frequently inaccessible. See this post.