The International Genealogical Index (IGI) was not completely available online until 2012. At that time, it was added to the FamilySearch.org Historical Record Collections. It is easily located by doing a title search from the Historical Record Collections search page. Here is a screenshot showing a search for the IGI.
The IGI search page has selections for searching either the Community Contributed IGI or the Community Indexed IGI or both. If I just select the Community Contributed IGI and do a search for one of my ancestors, here are the results. I will search for my Great-grandfather, Henry Martin Tanner, by entering his name in the search fields as shown below and unchecking the Community Indexed IGI.
Here are the results.
The results here illustrate that entries in the IGI can be duplicated a number of times. The first four results are references to my ancestor. There are also additional references to him down in the list. The FamilySearch search engine also picks up references to "Martin Henry Tanner" who is my cousin and a source of confusion with Henry Martin Tanner, my Great-grandfather. The duplicate entries represent multiple submissions of the same individuals. As a note, the IGI information has been included in the currently available FamilySearch.org Family Tree and the duplicate entries in the Family Tree came, in part, from this initial duplication.
At this point, the real issue is whether or not you can find a reference to the original document that was used to create the IGI entry. The best source for detailed information about the origin, structure, and content of all of the various forms of the IGI is the following book.
Jaussi, Laureen Richardson. Genealogy Fundamentals. Orem, Utah (284 East 400 South, Orem 84058): Jaussi Publications, 1994.
When you click on one of the results from the search illustrated previously, here is a sample of the page you might get.
The entry at the bottom of the page on the left with the arrow pointing is the IGI Film number. This number can be used to find the microfilm record of the "original" IGI microfilm roll. You can search with the numbers in the FamilySearch.org Catalog. Here is a screenshot of the results of such a search.
In this case, these original microfilms are in the Family History Library, Special Collections and have access limited to members with temple recommends and you will likely have some additional research to do with the original microfilm in your search for the source records.
From time to time, you may find one of the following types of entries in the FamilySearch.org Family Tree:
If you see this notice, then you should click on the link to learn more. You will then be taken to the following page:
Here is the link to read the previous posts in this series.