Refer to figure.
Engine RPM is a direct indication of power in piston engined aircraft, so the tachometer is a primary engine instrument that measures the rotational speed of the engine's crankshaft or the helicopter's main rotor in revolutions per minute, giving the pilot critical information on the engine's performance and helping to ensure that the aircraft is operating within safe and optimal RPM ranges.

Turbines rotate so fast that the numbers are too large to make sense of, so percentages are used instead (100% means full power), so engines can be compared easily. The RPM indicator (or tachometer) of a piston engine can include a small red arc within the green one used for continuous operation.

The engine (T) and rotor RPM (R) needles are near each other in the same dual tachometer or can be separated. In powered flight, the needles will be joined, in autorotation, they will be split, as shown in the figure.