To understand the I.G.I. you really need to understand how it developed and where to find it now via the familysearch.org website.
When I started my research in 1996 I was using the set of microfiches at Tameside Local Studies and Archives Centre when they were based at Stalybridge and at the Family History Centre, Patterdale Road. You can still use the microfiches but since it went online in 1999 mostly everyone began to use the www.familysearch.org website. For reasons of their faith the Mormons believe in the baptism of their ancestors by proxy into the Morman faith. For this purpose they have filmed millions of entries from Parish Registers and Bishop's Transcripts all over the world. This information was searchable on-line for everyone via the old Family Search site, but this has finally been turned off and you can only access it now via their new website
There are two separate parts to the I.G.I.
In general those entries from their controlled extraction programme can be regarded as accurate, making allowance for the fact that errors can happen when transcribing any original sources, but those entries from private patron submissions are to be viewed with extreme caution. The rules governing proof of ancestry for church members was not as rigorous as those required by family historians, and were indeed interpreted with a considerable degree of license, especially in the early days. Be especially careful with entries that say 'Relative' or 'About'. In the past church members went about collecting as many entries of the same surname in an area as possible, without proving any relationship between them. Also having found a marriage of their ancestors in a certain place they could assume by some guesstimate formula that she/he had been baptised in the same area about 20/25 years previously, again without any real proof at all.
A large number of records in the controlled extraction programme were taken from Bishops' Transcripts. This is because the Parish Registers at the time were still at the church and the incumbent may have refused permission to film, so the Mormons went along to the relevant County Record Office to film the BT's.
Although the I.G.I. was possibly one of the most useful tools available to family historians, it was vital that it was treated as an index only - it is not a primary source, but is extremely useful in trying to pinpoint ancestors for further research. There are, however, many pitfalls and without referring back to the original parish registers and or bishop's transcripts, it is extremely easy to find that you are researching the wrong line altogether. You can get very useful leads from the I.G.I., but the parish registers often contain the extra information required as proof: examples:
So without referring back to the original registers, you can also be missing out on vital extra information.
Family Search’s website now the main route into their records, which continue to be added to at an amazing rate, some are indexed and some are browse only. It’s a good idea to have a browse around the collections before you begin searching, but if you do want to simply search the old IGI you can go to the following pages on their website.
You may want to try and find out if your ancestor had any brothers or sisters. Here's an example from my own family history Go to the www.familysearch.org/search/collection/igi I.G.I. searchscreen:
Input the surname only CARTWRIGHT
Then click on birth and enter birthplace: CHESHIRE
A range of dates: 1790-1820
Then parents: Richard CARTWRIGHT and mother Ann
This should reveal five baptisms of children belonging to Richard and Ann Cartwight - four of these at Gee Cross, Hyde and one at Stockport St Mary's.
All entries on the I.G.I. were added in a series of batches each with their own individual number, called batch numbers. There are two valuable websites to help with this, one was created by Hugh Wallis. He organized the batch numbers for each country, county, city and parish (British Isles and North America), so that you can easily see which batches are available for the area you are researching. Steve Archer at Archer Software has done the same thing but, has included extra batches that didn't make it onto the Hugh Wallis website.
Try just adding a SURNAME and a batch number only - this will reveal all the events related to that surname for the period covered by the batch number. Again an example from my own family history. Batch number C076442 covers the christenings at Old Chapel Dukinfield between 1676-1713 and 1762-1828 and surname STANLEY, this gives me 154 results and, yes, they are all mine and they were a bit of a nightmare to sort into family groups to add to my tree.
We are quite lucky in Tameside in that most of the parish registers in the area had been filmed and are on the I.G.I. up until at least 1837.
It is very much still a work in progress and learning the best search technique is also evolving. So far I am finding that if you initially use a wide search criteria on the less is more principle, you can adjust your criteria at any time by clicking on the little down arrow next to REFINE YOUR SEARCH in the top left-hand corner. It does seem to be rather PLACE sensitive. Family Search will then give you the results which match your criteria and below these a list of near matches. You can still search for siblings on the new site with a similar process to the I.G.I. and you can still use the batch numbers from Hugh Wallis’s and Steve Archer's websites.
Remember, though that some of the new records lead you to actual images of the original source material whilst others are browse only. There are also new records on the site which are indexes only to various census and nonconformist records, but these will take you through to pay sites to view the images – fine if you already have a subscription. The site is adding millions of new records world wide, so continually going back to the old I.G.I. means you are missing out on millions of new records. Also burials are appearing for the first time – these were largely ignored when compiling the original I.G.I.
An interesting bit of extra help from the Family Search is while searching censuses, particularly 1911. Find My Past does not include place of birth in their search results, but Family Search does. This can save quite a bit of time when searching for common names.
Family Search Good and growing coverage of C of E parish registers for the Cheshire Towns of Tameside. I can't be exact, but it seems that most of these are covered from the inauguration of the particular churches records until about 1906. These are indexes only, but most of the original films can be consulted at Tameside Local Studies and Archives Centre - click here to go to their church register holdings.
Non Conformist Catholic Churches:
They are also availabe via Find My Past, Cheshire Record Office did a deal with FMP to release the images of the indexed Family Search Cheshire Collection.
The B.V.R.I. - was a collection of indexed Baptisms and Marriages as an addition to the I.G.I. (International Genealogical Index). It included approximately 17 million christening entries and 7 million marriages . It does include quite a bit of extra post 1837 information on baptisms and marriages at Ashton St Michael, Stalybridge St Paul's, Denton Christ Church and St Lawrence, and Dukinfield St John and St Marks. Some of this is up until about 1910 so is very useful for narrowing down some information before purchasing certificates via GRO or Tameside Register Office. The marriage entries do give both father's names so pretty useful.
As far as I can deduce all the previous I.G.I. records and the B.V.R.I. databases are now all on the new Family Search website.
If you go to familysearch.org and then Browse their UK and Ireland collections you will find lots of new databases, which are being added to all the time.
For more information see Family Search WIKI
Understanding the IGI Although written from an American perspective - lots here to explain the history of the International Genealogical Index (IGI)