Why the code is IL
Illinois got the easiest treatment available: letters one and two of the state name, untouched. When the Post Office Department published its fixed two-letter list in October 1963, the goal was to make city, state and ZIP Code fit on a single machine-read line, and any name that could spare its first two letters without colliding with another state kept them.
Four states begin with I, and three of them cleared that bar: Idaho took ID, Illinois took IL, Indiana took IN. Iowa is the lone exception. IO was available on paper, but Iowa was given IA, its first letter and its last. That makes the I block far kinder than the M block, where eight states have to share one initial. Learn one exception and the I group is finished.
The name itself is a French spelling of an Algonquian one. French missionaries and traders wrote the name of the Illiniwek people as Illinois, and the French -ois ending is why the final S stays silent in English. The postal code sidesteps that entirely, which is why plenty of people who hesitate over the spelling still type IL without a pause. Before 1963 the standard short form was Ill., period included, and it survives in older court citations.
Codes people confuse with IL
| Code | State | Why the mix-up happens |
|---|---|---|
| IN | Indiana | Shares a border and an initial with Illinois. IN is also the English word "in", so address parsers and spreadsheet cleaners strip it or autocorrect it. |
| ID | Idaho | ID reads as "identification" everywhere else. A spreadsheet column headed ID almost never means Idaho, so people distrust the whole I block. |
| IA | Iowa | The one I-state that skips its second letter. Once you know Iowa breaks the pattern, you start second-guessing whether Illinois does too. |
| IL | Israel (country) | IL is also the ISO country code for Israel and the .il domain. Forms that merge country and state lists can offer IL twice. |
| Il / lL | Nothing, a typo | Capital I and lowercase l are identical glyphs in most sans-serif fonts. Copied text quietly turns IL into a lookalike that fails validation. |
That last row is worth guarding against. USPS addressing standards ask for the whole address in capitals with no punctuation, and typing IL in uppercase by hand rather than pasting it is the cheapest way to be sure the two characters are the ones the sorter expects.
Quick facts about Illinois
| Capital | Springfield |
| Largest city | Chicago |
| Region | Midwest |
| Statehood | 1818, the 21st state |
| Population band | Over 10 million |
| Area rank | 25th of 50 |
Where you will see IL
- Mail. The code closes the last address line: Springfield, IL and Chicago, IL. Anything longer than two characters slows the sorter down.
- Data files. Federal datasets carry Illinois as FIPS code 17 and as ISO 3166-2 code US-IL. Both appear in census and election exports.
- License plates. Illinois plates spell the state name out and carry the Land of Lincoln slogan. The two-letter code lives on the registration paperwork, not the plate.
- Sports. Rarely. Every major Illinois franchise is branded Chicago, so broadcast graphics and box scores show CHI, and the state code never surfaces.
- Crosswords. Clued as "Land of Lincoln: Abbr." or "Chicago's state: Abbr." when the grid needs two letters.
- Airline and hotel bookings, never. Travel systems use three-letter airport codes, so a Chicago trip is quoted as ORD or MDW and the state code stays out of it.
Stop second-guessing the I block
Statedoku builds each daily puzzle around state codes, capitals and regions. Five minutes and IL, IN, ID and IA stop blurring together.
Play today's puzzle →