Washington, D.C., is located just west of 75° west longitude and is located in UTM zone 18 (72° west longitude to 78° west longitude). St. Louis, Missouri, is located just west of 90° west longitude and is located in UTM zone 15 (90° west longitude to 96° west longitude). San Francisco, California, is located just west of 122° west longitude and is in UTM zone 10 (120° west longitude to 126° west longitude).
UTM coordinates are recorded in meters. The UTM northing coordinate (the Y coordinate) for a feature is the distance in meters north from the equator to the feature. The UTM easting coordinate (the X coordinate) for a feature is the distance in meters east or west from the central meridian of the UTM zone. The central meridian for UTM zone 10 (with boundaries at 120° west longitude and 126° west longitude) is 123° west longitude. UTM eastings are prevented from having both positive values (meters east of the central meridian) and negative values (meters west of the central meridian) by setting the UTM easting for the central meridian at 500,000 m. A feature located 200,000 m west of the central meridian for a UTM zone would have an UTM easting coordinate of 300,000 m. A feature located 200,000 m east of the central meridian for a UTM zone would have a UTM easting coordinate of 700,000 m.
Many maps display a UTM grid as light blue or black lines. The UTM zone for the map will be reported in the map legend. The UTM system is just one example of a map projection. Often data within a GIS will be from various map projections. Snyder (1987) created an excellent reference to map projections.