Footnotes ...

<1>	When there's a risk of an area code running out of phone numbers, an 'Overlay Area Code' is created that has the same geographic boundaries as the existing area code. In this case, the 309 code has been Overlayed with the 861 area code. New phone numbers in the Illinois City area will be assigned with one of these codes: 309 or 861. As a result, placing a call in the Illinois City area will require 10-digit dialing (where you enter both the area code and then the phone number).
<2>	ZIP Codes have been created by the United States Postal Service (USPS) as a way of grouping addresses to make delivery more efficient. The boundaries of the ZIP Code can be changed, re-assigned, eliminated or overlapped with other ZIP Codes. So keep in mind that any given ZIP Code is only 'loosely' associated with a community (such as Illinois City). When looking for a ZIP Code, whether Illinois City or elsewhere, it's always best to check the website of the United States Postal Service (USPS).
<3>	The elevation is an estimate for the area around Illinois City - it will be higher in some places and lower in others.
<4>	Longitude & latitude values may not be the geographic center of Illinois City, rather it may be a prominent intersection or the center of population, etc. It will vary depending on the source.
<5>	Name of the USGS 7.5 Minute topographic map which includes Illinois City.
<6>	Every year, the US Census Bureau creates a population estimate for the preceeding year. Keep in mind that this value is an estimate and not an actual count. The most recent Census estimate is for the year 2019.
<7>	The area value has been supplied by the US Census Bureau and it can be different than the geographic boundaries of Illinois City. The value given by the Census Bureau reflects their statistical (or sampling) requirements. For example, if there is a cluster of people living just outside of the community's official boundary, the Census-defined area might be expanded to include that group.
<8>	A Housing Unit is a broad term for those places where people live (whether occupied or temporarily vacant), such as a house, an apartment, a mobile home, or an area that is considered as a separate living quarter. Separate living quarters (consisting of one or more rooms) are considered to be those places where people live, sleep and eat separately from others and that have access to the outside (either directly or by a shared hallway).