For example, there might be two layers of aluminum, each of which can be used for making wires. One way to arrange the wires in a chip is to have a set of horizontal wires in one layer and a set of vertical wires in the other. In a VLSI chip, everything is essentially planar. In addition, wires have a width as well as a length and so take up area. The two important parameters for a VLSI designer are speed and area.
Area is limited, of course.
In addition, larger chips tend to have more defects, and so are more expensive to make. The chip is a convex region R of the plane. The time to propagate along a wire is proportional to the length of the wire. Storage for one bit takes area at least l 2 . 7. Each input bit is input only once.