The Kentucky Geological Survey uses the AAPG stratigraphic code framework to designate formal stratigraphic units and miscellaneous informal rock unit names. The AAPG code is an alphanumeric string eight characters long divided into two parts. The first part is a three digit numeric code that places the code in a gross stratigraphic framework: the first digit represents the era, the second represents the system, and the third digit represents series. The second part is an up to 5 character mnemonic for the specified unit. Usually, the fifth character is reserved for a modifier as in "1" for first, "M" for middle, "U" for undifferentiated, and others.
A four digit "sequencing suffix" was added to each stratigraphic code. This suffix when combined with the original three digit code allows sorting the codes in detailed stratigraphic order. Units with the same stratigraphic sorting sequence are equivalent. "Equivalent" may imply a member is stratigraphically at the top of the formation including it.
Stratigraphic codes for the Mississippi Embayment, Illinois Basin, Mississippian Plateau, Cincinnati Arch, and Appalachian Basin geologic provinces are included in the same list. Units in different provinces with the same stratigraphic sorting sequence are roughly correlative. Informal units and drillers' terms are given sequence codes that imply stratigraphic relations of "above/younger" or "below/older"; in many cases strict correlation with formal units is not possible.