The website states the following:
"The sources for this index include original county clerks’ marriage records, such as marriage registers and licenses, as well as publications of county genealogical societies and private individuals. For each marriage, the index includes the names of the groom and bride, the date of the marriage or issuance of the license, the name of the county in which the marriage was performed, and a citation to the original record—volume and page number for registers or file number for licenses. Volume and page or license numbers are omitted in the index when published sources failed to provide this information.The site is easily searched by typing in the name of either the bride or the groom. This site contains only the index with references to the licenses numbers or original record volume; no images are included on this site. Having said that, this index is still a valuable source of information if you have ancestors that were married in Illinois. A link is provided on the site to order a copy of the record as well. If you would like to search this database click here. Have fun searching!
Although the primary aim of the project is to enter marriages occurring prior to 1901, some volunteers have chosen to enter a county's marriages through 1920. Click here for a listing of counties and date spans currently included in the Illinois Statewide Marriage Index."