Cross-hatching

Hatching (hachure in French) is an artistic technique used to create tonal or shading effects by drawing (or painting or scribing) closely spaced parallel lines. (It is also used in monochromatic heraldic representations to indicate what the tincture of a "full-colour" emblazon would be.) When lines are placed at an angle to one another, it is called cross-hatching.

Hatching is especially important in essentially linear media, such as drawing, and many forms of printmaking, such as engraving, etching and woodcut. In Western art, hatching originated in the Middle Ages, and developed further into cross-hatching, especially in the old master prints of the fifteenth century. Master ES and Martin Schongauer in engraving and Erhard Reuwich and Michael Wolgemut in woodcut were pioneers of both techniques, and Albrecht Dürer in particular perfected the technique of crosshatching in both media.

Artists use the technique, varying the length, angle, closeness and other qualities of the lines, most commonly in drawing, linear painting and engraving.

The main concept is that the quantity, thickness and spacing of the lines will affect the brightness of the overall image, and emphasize forms creating the illusion of volume. Hatching lines should always follow (i.e. wrap around) the form. By increasing quantity, thickness and closeness, a darker area will result.

An area of shading next to another area which has lines going in another direction is often used to create contrast.

...
Wikipedia