Illinois has 102 counties across 1,575 postal codes — the fourth-highest county count in the country, behind Texas, Georgia and North Carolina. At roughly 15 codes per county it sits in the middle of the national range, which makes it a good default when you want a county-keyed fixture that is neither trivially small nor Texas-sized.
The prefix distribution is where the state surprises people. The largest by code count is 628, whose principal place in the postal file is Albion — a town of about two thousand people in the far south of the state, closer to Kentucky than to Chicago — ahead of 601 around Schaumburg and 604 around Joliet. Chicago itself is 606xx, and the city's codes are a small share of the state's total.
So a fixture set drawn from Chicago covers one prefix out of 29 and none of the downstate sorting geography, where a single place name can serve a wide rural area and delivery economics are entirely different. If your shipping estimates, delivery windows or density-based logic vary at all, the 61x and 62x prefixes are the half that will show it.
The prefixes are also not contiguous — 600 to 629 with gaps — so Illinois needs a set rather than a range, unlike the 25 states that can be expressed as one. It is the same distinction Maryland runs into, and the reason a prefix rule written against a tidy state does not generalise.
One last trap, and it is a matching bug rather than a geography one. Illinois has 84 postal codes whose place name is exactly Chicago, and 87 whose place name begins with it — the extra three are Chicago Heights and Chicago Ridge, separate municipalities in Cook County with their own governments and their own tax rates. Any query written as a prefix match or a LIKE 'Chicago%' quietly folds three suburbs into the city, and any dashboard built on it reports Chicago numbers that are not Chicago's. Match place names on equality, and if you need the metro, use the county or the code.
