Cross-contours are often used when hatching. The cross contour lines may be carried all the way around the form, or used in small sections, curved, or straight, as in this example. The angle of the hatching as it moves around the form changes according to perspective.
Even if you are using shading, and attempting to create a smooth surface, being aware of the flow of cross-contours as you draw can help you create a shaded surface that follows and enhances the three-dimensional form, rather than fighting against it.