PLSS Maps

   PLSS maps are Public Land Survey System Maps. It is a surveying method. It is used to survey and spatially identify land parcels before ownership is designated.

                                         http://nationalatlas.gov/articles/boundaries/a_plss.html

 
The North and South line runs through the initial point. It is known as the true meridian and it is called the Principal Meridian. There are 37 Principal Meridians. Each Principal Meridian is names and is used to distinguish the various surveys.
The East and West line runs through the initial point. That initial point is called the Bare Line. This line runs perpendicular to the Prime Meridian.