In analog circuits, we think something about voltages on terminals ranging over natural values. This provides us the freedom to make an enormous variety of digital circuits, but sometimes that freedom actually creates the design function more difficult. To developed ever more complex digital circuits, we may move to a much harder, digital abstraction, in which the voltages on the ends are thought of as only taking on quantities that are either "low" or "high". This PCAP machine is build up of a basis set of components, called gates, that are made out of simpler analog circuit elements, such as resistors and transistors. Adopting the digital abstraction is a big limitation on the types of circuits that can be created. However, digital circuits may be easy to develop and create and also can be inexpensive because the basic components are simple, versatile, and combinations are easy to think about. These characteristics allow designers to create incredibly complex systems by designing small parts and giving them together into increasingly larger pieces. Digital calculators, watches, and computers are all developed this way.