Hatching (hachure in French) and cross-hatching are artistic techniques used to create tonal or shading effects by drawing (or painting or scribing) closely spaced parallel lines. When lines are placed at an angle to one another, it is called cross-hatching.

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

Technique

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The main concept is that the quantity, thickness and spacing of line will effect the brightness of the over all image. By Increasing quantity, thickness and closeness, you'll have a darker image.
It's often a good idea to contrast an area of shading with another patch which has lines going in another direction- thus creating contrast.
Various line work can be used to represent colours, and it's often a good idea to use the same type of hatch to represent certain tones (i.e. red might be made up of lightly spaced lines, whereas green could be made of two layers of perpendicular dense lines), resulting in a realistic image.

Reference

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• "hatching" article in ArtLex Art Dictionary

Artistic techniques

Schraffur