Lead systems have been developed based on theoretical considerations and references to anatomic landmarks that facilitate consistency among individual patients (eg, standard 12-lead system). Einthoven established electrocardiography using three extremities as references: the left arm (LA), right arm (RA), and left leg (LL). He recorded the difference in potential between the LA and RA (lead I), between the RA and LL (lead II), and between the LA and LL (lead III) . Because the signals recorded were differences between two electrodes, these leads were called bipolar.
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