Although Indiana was organized into the United States since the Northwest Ordinance in 1787, its land was not always available for settlement.

The Vincennes Tract, Clark's Grant and an area known as "The Gore" in southeastern Indiana (resulting from the Treaty of Greenville 1795) existed during the Northwest Territory.

The remainder of Indiana land was acquired by Indian Removal Act and purchases by treaty between 1804 and 1840.

The oldest counties are generally in the south near the Ohio River, whereas newer ones were in the north in territory acquired later.

[6] In addition these codes are also used by INDOT for structure IDs on objects such as bridges and overpasses.