Cross-hatching is a way of filling part of a drawing with lines. You draw one set of close lines, then another set across them, so the lines cross. Artists use it to make a patch look darker — more crossing, more shade — without painting the whole area in. In diagrams and plans, the same kind of crossing lines can mark that a part is a different material, or that you are looking at a cut-through view. It is a technique, so people often speak of “the cross-hatching” on a picture as a mass of lines, not as one single stroke. Though actually, a single set of parallel lines is just hatching; it becomes cross-hatching when a second set cuts across the first.