Electronics refers to the flow of charge through semiconductors, whereas electrical refers to flow through metals. This field was originally called "Radio techniques" until 1950, as its main application was radio transmitters and receivers. Digital circuits use Boolean algebra and are the basis for digital computers, typically using two voltage levels of "Low" and "High" to represent 0 and 1. Most analog circuits are built from basic amplifier circuits and use a continuous range of voltages, though the distinction between analog and digital can be blurred as they both use linear and non-linear elements.