Portolans share another characteristic: a network of lines, like the web of a spider, that forms a grid for the map. This grid can easily be seen by looking at the portolan chart from the blank side, against the light, since the vellum is fairly transparent. The hole at the center of the circle which defines the grid is also visible from the back of the portolan chart. Directions are indicated on the portolan charts by this network of lines joined on the points dividing the circle that is a basic element of the portolan chart. The circle is divided into sixteen equal parts, each joined to others through the center point on the opposite side. The lines are called "rhumb lines" or simply "rhumbs."

The rhumbs for the eight primary winds (or directions) are drawn in black (or sometimes gold) ink; the eight half-winds are in green; the sixteen quarter winds in red. Patterns of squares, triangles and parallelograms are visible within the circle on portolan charts.