"This publication of Illinois state census schedules for 1820 is a companion work to the 1810 federal and 1818 territorial censuses published in another volume of the Illinois Historical Collections. The introduction to the earlier returns presents the history of census taking in Illinois and gives tables and a map showing the population by counties in 1818 and 1820. This book contains not only the 1820 state census schedules but also notes comparing all discrepancies between the names as written in the 1818 territorial, the 1820 state, and the 1820 federal censuses. While in most instances these represent merely minor variations in spelling, there are enough marked discrepancies to indicate that all three sets of returns are necessary to compile a correct list of the pioneers of each county. Symbols have been used in both volumes to indicate names appearing in the other two censuses. In making this comparison families were not assumed to be identical unless they appeared as residing in approximately the same vicinities. Similarity of names alone was not considered sufficient unless they were quite distinctive or unless other evidence corroborated the hypothesis. Names appearing in the federal but not in the state census of 1820 are given in the notes." Census data is included for the following counties: Alexander, Bond, Clark, Crawford, Edwards, Franklin Gallatin, Jackson, Jefferson, Johnson, Madison, Monroe, Pope, Randolph, St. Clair, Union, Washington, Wayne, and White. A full-name index completes this work.