N or RL or A1/A2/M1/M2 is the mysterious 3rd electrode when measuring biosignals such as ECG, EEG or EMG but do we need it? Yes we need it but it doesn't matter where we place the electrode. This electrode in technical terms is called GND and is there for pure technical reasons because amplifiers always measure against GND. However in biosignal applications GND is cancelled out by calculating the difference between two amplifier signals – both measuring against GND. So it doesn't matter where the GND is placed on the body. However, it reduces 50Hz/60Hz mains interference and in the old analogue days GND actually injected an inverted mains signal in the body to eliminate it even more and people called it rather N for neutral. Nowadays the main interference can be filtered out by a digital filter as shown in the 2nd trace in the clip and much less effort has to be put into the analogue design which won't require perfect mains cancellation. One can even measure an ECG with an amplifier which measures against GND and then do heavy digital filtering to remove the mains interference.
GND has different labels on the elctrode connectors depending on the signal measured but from the electronics point of view it's always the same:
Recorded with the Attys.
next: ECG artefacts