Use IL as the Illinois abbreviation whenever you write an address or fill in a form. The USPS gave the state this two-letter code: a capital I, a capital L and no period. The traditional written form is Ill., which you may still meet in newspapers, legal citations and older books.
Each one has its own place. Postal workers, sorting machines and online forms expect IL. Readers of a paragraph usually see the full name, Illinois, and Ill. shows up mostly in short entries where space is tight.
Quick facts
Postal code
ILTraditional abbreviation
Ill.AP style
Full name in stories; Ill. in datelines and listsPeriod
No period in IL; Ill. ends with oneCapital
SpringfieldStatehood
1818, the 21st stateNot to be confused with
IN (Indiana), IA (Iowa), ID (Idaho)
Which Illinois abbreviation to use where
| You are writing | Use | Sample |
|---|---|---|
| An envelope or package | IL | Peoria, IL 61602 |
| A web form or spreadsheet | IL | State: IL |
| A news dateline or city list | Ill. | Naperville, Ill. |
| A normal sentence | Illinois | We moved to Illinois in June. |
That small chart answers most questions. The code is for systems, the old form is for compact print, and the full name is for prose.
What is the abbreviation for Illinois on an envelope?
It is IL, and it belongs on the final address line, after the city and a comma. A single space then separates IL from the five digit ZIP.
Return address on a birthday card:
Priya Shah
45 Lake View Road
Evanston, IL 60201
Notice the two capitals and the missing period in IL.
The Postal Service asks for this format because scanners read it best. Writing "Ill." or "Illinois" on mail is not wrong, and the card will still arrive, but the code is the standard.
Ill. in news and formal writing
Before two-letter codes existed, writers used longer short forms, and Illinois got Ill. AP style, which most American newspapers follow, now spells out state names inside the body of a story. Ill. survives in datelines, photo captions, sports scores and lists of towns.
Legal writing also uses Ill. in case citations. In a school report, though, the plain word Illinois is best unless your teacher says otherwise.
One small quirk: Ill. looks like the word "ill," meaning sick. The capital letter and the period are what tell a reader it is the state.
Keep IL, IN, IA and ID straight
IL has three look-alike codes that also open with I, all easy to swap when typing fast:
- IL is Illinois.
- IN is Indiana, the neighbor to the east.
- IA is Iowa, the neighbor to the west.
- ID is Idaho, far to the west.
Another trap is the letter shape. In some fonts a capital I and a lowercase l look almost the same, so "Il" or "lL" can sneak into a form. Type IL in two capitals to be safe.
Illinois at a glance
Illinois became the 21st state in 1818. Springfield is the capital, while Chicago is by far the largest city. The state calls itself the Land of Lincoln. French explorers wrote down the name of a Native American people living along the rivers there, and the spelling stuck, which is why the final s is silent: you say "ill-uh-NOY."
Codes for the other 49 states are gathered on the state abbreviations page.
Simple rules to remember
- Two capitals, no period, only in addresses and forms: IL.
- Capital I, two lowercase l's, a period, only where a style guide asks: Ill.
- Full name everywhere else.
- Never write "I.L." with periods between the letters.
For the general rules on periods and capitals in short forms, read acronym vs abbreviation.
Questions people ask
Which code does the post office use for Illinois?
It is IL. Mail sorting relies on it, and so do online forms and spreadsheets.
Is IL Illinois or Indiana?
IL is Illinois. Indiana is IN, and the two neighbors are a common mix up.
Why is the s in Illinois silent?
The name came into English through French, and the French spelling kept an s that is not spoken. People say "ill-uh-NOY."
Is Ill. still used?
Yes, but less often. It lingers in some datelines, lists, captions and legal citations, while addresses use IL.
Can I write Illinois in full on an envelope?
Yes, mail will still be delivered. The two-letter code IL is preferred because sorting machines read it more reliably.