Voltage is always measured between two points in a circuit. This is true whether using a voltmeter or an oscilloscope. When an oscilloscope probe touches a point in a circuit, a waveform usually appears on the display, even if the ground lead is not connected. In this situation, the reference for the measurement is conducted through the safety ground of the scope chassis to the electrical ground in the circuit.

By virtue of their two probes, digital voltmeters measure potential between two points. Because they are isolated, these two points can be anywhere in the circuit. This has not always been the case. Before the advent of the digital voltmeter, hand-held meters known as VOMs (Volt-Ohm-Meters) were used to measure "floating" circuits. Because they were passive, they tended to load the circuit-under-test. Less invasive measurements were made with the high-impedance VTVM (Vacuum Tube Volt Meter). The VTVM had one major limitation - the measurement was always referenced to ground. The VTVM housing was grounded and connected to the reference lead. With the introduction of solid-state gain circuits, high performance voltmeters could be isolated from ground, allowing floating measurements to be made.

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