The Landmarks Survey (a handle for the "Illinois Historic Sites Survey Inventory") was conducted on a county-by-county basis. Surveyors first researched the history of each county to search for references to buildings or other landmarks associated with important events and people. They then enlisted the assistance of local historical organizations in identifying and locating the county's important resources. Each site was then visited, and if the building was still standing, a two-page inventory form was filled out and a 5-by-7-inch photograph taken. The kind and amount of information collected on each property varied; some inventories recorded only the location, ownership, construction date, and a short entry on a structure's history, while others included in-depth histories provided by the local historians. Some 12,000 properties were inventoried in 102 counties.