They create a pattern of their own that does not exist in either of the originals. The result is a series of fringe patterns that change shape when as the grids are moved relative to each other. The word Moir was first used by weavers and comes from the word mohair , a kind of cloth made from the fine hair of an Angora goat.
Artists hate moir patterns because they can sometimes ruin a printed photo or drawing.
But they can also be great fun. This can happen, for example, when scanning large areas of what seems to be a single colour. Sometimes what we perceive is a block of colour, is actually a great many dots of ink. When printed, small random variations in value can occur due to the imperfections of print technology. The grid pattern of the scanning process combined with these almost-invisible random ink patterns can produce unexpected results.