Fundamentally, all oscilloscopes measure voltage (in the Y axis) against time (in the X axis) and all voltage measurements are made between two points. Most oscilloscopes are designed with the expectation that one of these points will be fixed firmly to ground, and so the reference lead of the probe connects electrically to the shell of the BNC input, which connects to the chassis, which is grounded through the mains lead. This presents a problem that most oscilloscope users encounter regularly: many voltage measurements are not referenced to ground. In fact differential or floating voltages are everywhere and there are many solutions to the problem, though some are more suitable than others. More information on Differential and floating voltage measurements >>