Peripheral leads of a 12-lead electrocardiogram recorded in a healthy young participant in a drug trial with right arm–left arm cable reversal (top panel) and with correct cable connections (bottom panel). In this one and in the subsequent figures, the positions of the peripheral electrodes and their effect on Einthoven’s triangle are schematically presented (in this figure to the right of each ECG). The cable reversal can be recognized by the negative P–QRS in lead I and positive P wave in lead AVR. Note, however, that the frontal QRS axis is very similar in the ECG and without cable reversal (96 and 86°, respectively). See the text for details.