Percival Boyd's compilation of marriage entries in English parish registers from the beginning of registration in 1538 up to 1837, when civil registration began. The index may be consulted at the Society of Genealogists (which has published a Key) or at the Guildhall Library, London. Registers from over 4 300 parishes have produced an index of over 7 million names. However, as the entries are copied mainly from printed registers, coverage is not complete. The Index is particularly useful for researching East Anglian ancestry, for over 95 per cent of the ancient parishes are included, in contrast to the limited coverage of the International Genealogical Index.
http://www.originsnetwork.com/help/popup-aboutbo-bmi2.htm Marriage Index.