Digital electronics deals with the electronic manipulation of numbers, or with the manipulation of varying quantities by means of numbers. Because it is convenient to do so, today's digital systems deal only with the numbers 'zero' and 'one', because they can be represented easily by 'off and 'on' within a circuit. This is not the limitation it might seem, for the binary system of counting can be used to represent any number that we can represent with the usual denary (0 to 9) system that we use in everyday life.