Section ranges are east or west measurements taken from a designated principal meridian, while township ranges are north or south measurements taken from a designated base line. The two measurements form a grid noting the townships. The procedure is known as the Public Land Survey System.
Each township measures 6 square miles, which is the dimension used for maps in the National Atlas. Townships can be further divided into square mile sections and then into quarter sections. Permanent markers are placed at a section's corners.
Each 6-square-mile township is labeled by a north or south township designation and an east or west range designation. For example, a township labeled Township 7 North, Range 2 West would be in the seventh row of townships north of the baseline and in the second column of townships west of the principle meridian.
This PLSS, an idea from Thomas Jefferson, dates back to shortly after the Revolutionary War. The government had new territories west of the original 13 colonies and wanted to divide up the land to give to Revolutionary War veterans. They also needed to sell off some land to raise money to run the new country. The system is virtually the same today, but public domain lands, such as National Parks, are exempt.