The International Genealogical Index is often referred to as the IGI or Mormon Index.It is a name index and contains millions of names gathered from a number of sources all over the world, many of them taken from parish registers as part of an organised program of careful transcription, others provided by individual and not always overly careful researchers.

Although you will need to check the original sources of the information contained in the Index, you will often find that the Index can be a great help to your research. However, its coverage is far from complete, so the fact that the ancestor you are seeking does not appear in the IGI should not cause you to give up.
For the British Isles, the primary source of names has been the records of baptisms, marriages and deaths in the parish registers, together with a number of nonconformist registers. Coverage is not comprehensive.The main use of the IGI is searching for people before Civil Registration began (1837 in England and Wales; 1855 in Scotland; 1864 in Ireland). The main arrangement is by pre-1974 counties.The Middlesbrough Reference Library holds the following editions of the IGI on microfiche:

English counties 1992
Isle of Man 1992 (an edition arranged for the whole of the Isle of Man in one alphabetical list by surname)
Channel Islands, including Alderney, Guernsey and Jersey 1992
Wales 1984 (this edition is arranged for the whole of Wales in one alphabetical list by surname)
Wales 1992 by counties
Scotland 1981 (this edition is arranged for the whole of Scotland in one alphabetical list by surname)
Scotland 1992 by counties
Ireland 1992 (an edition arranged for the whole of Ireland in one alphabetical list by surname)
Ireland 1992 by counties
World by individual countries 1988

The data heretofore published on microfiche is now being incorporated and progressively extended on the Mormon web site Family Search (http://www.familysearch.org) where it forms one of a number of databases on this web site.A folder in Middlesbrough Reference Library-IGI Instructions and Regions- is located by the cabinet holding the microfiche.
 
 

To understand how the Mormon Index has come into being, you have to know about the beliefs of the Mormons or members of The Church of Jesus Christ of Latter-day Saints. For them, life does not end at death. At death, their eternal spirits go to a spirit world, where they continue to learn while they await the Resurrection and Final Judgement. Members of the Church believe that the family can also continue beyond the grave, not just until death.

This is possible when parents and their children make special promises, called covenants, in sacred temples. These covenants, when made with the authority of God and faithfully kept, can unite families for eternity.

Members of the Church believe that their deceased ancestors whom they love and desire to serve can also receive the blessings of being eternally united with their families. For this purpose, Church members make covenants in temples in behalf of their ancestors, who may accept these covenants, if they so choose, in the spirit world. In order to make covenants in behalf of their ancestors, members must first identify them. The Church of Jesus Christ of Latter-day Saints has gathered genealogical records from all over the world. These records are the pre-eminent source of genealogical information in existence comprising the Family History Library of the Church of Jesus Christ of Latter-Day Saints available at the Family History Library in Salt Lake City, Utah, and at Family History Centers throughout the world,
There is an online catalogue of this immense library, and there are Family History Centres which are in effect branches of this library in many towns and cities throughout the British Isles and indeed the world where microfilm copies of most of the library's holdings can be viewed. A list of Family History Centers is available at http://www.familysearch.org/Eng/Library/FHC/frameset_fhc.asp