Because of the shape of the earth, meridians converge toward the poles.

If not corrected, a township on the base line would be larger than one further north. To adjust for this problem, correction lines, called guide meridians, are established.

Guide meridians extend true north and south, generally at intervals of 24 miles to the east and to the west of the principal meridian.

The vertical lines to the east or west of the principal meridian are known by either a name or a number, such as, 1st Guide Meridian East, or 1st Guide Meridian West, or 2nd Guide Meridian West, and so forth.