Electricity flows from a battery in one direction only, and some components work only if the flow through them is in the correct direction. This acceptance of a one-way flow is called polarity. On most cars the negative () battery terminal is earthed and the positive (+) one feeds the electrical system.
Modern cars often need room for many wires in confined spaces. Some manufacturers now use printed circuits instead of bundles of wires, particularly at the rear of the instrument panel.