Planar coordinate systems, also called projected coordinate systems, convert the Earth’s curved surface to a flat Cartesian grid so that distances, angles, and areas can be expressed in meters or feet rather than degrees. Because every projection distorts area, shape, distance, or direction, planar systems limit zone width and adjust scale factors to keep those errors small. Unlike geographic coordinate systems (GCSs), which locate positions with latitude and longitude angles on an ellipsoid, planar coordinate systems (PCSs) replace values with linear x- and y-coordinates after a map projection has been applied. To show how different PCSs balance accuracy and usability, this entry uses the United States as a comparative case study, examining three widely used systems that reveal key differences relevant to map users worldwide. The Universal Transverse Mercator (UTM) grid supplies global coverage in sixty 6-degree zones and underpins most GPS/GNSS workflows. The State Plane Coordinate System (SPCS) delivers sub-meter precision tailored to individual states for surveying, engineering, and cadastral mapping. The U.S. National Grid (USNG) further simplifies UTM coordinates into concise alphanumeric references for emergency response and public safety.