The County's system of tax maps is based on the Illinois Public Land Survey System (PLSS). In the PLSS, each township is divided into 36 sections, and each section into four quarter-sections. A quarter-section is further divided into two tax map sheets, often called "pages". Each tax map (1/4 mile by 1/2 mile) represents the east or west half of one quarter-section, and typically there are eight tax maps per section.

The Public Land Survey System (PLSS) forms the foundation of Cook County's cadastral system for identifying and locating land records. Tax parcels are identified using township and section notation, in a modified format. This PLSS feature data set is intended to correspond to tax pages in the Cook County Assessor's Tax Map book (current as of tax year 2000 for 66% of the County and as of tax year 2001 for the remaining 33% of the County), and should not be used for measurement or surveyor purposes. In addition, the parcel attributes PINA (area/township) and PINSA (subarea/section) do not necessarily correspond to the PLSS township and section polygon in which a given parcel resides. The PLSS data is modeled as a single composite network coverage that encompasses townships (area), sections (subarea), quarter sections, and half quarter section.