Why is it called a 12 lead ECG, if it only has 10 leads?
For ECG recording, 10 electrodes are used but at each level voltage difference between two electrode points is measured and the combination the two points is termed lead. Read on for further clarification....

The term "lead" in electrocardiography causes much confusion because it is used to refer to two different things. In accordance with common parlance the word lead may be used to refer to the electrical cable attaching the electrodes to the ECG recorder. As such it may be acceptable to refer to the "left arm lead" as the electrode (and its cable) that should be attached at or near the left arm. There are usually ten of these electrodes in a standard "12-lead" ECG.

Alternatively (and some would say properly, in the context of electrocardiography) the word lead may refer to the tracing of the voltage difference between two of the electrodes and is what is actually produced by the ECG recorder. Each will have a specific name. For example "Lead I" (lead one) is the voltage between the right arm electrode and the left arm electrode, whereas "Lead II" (lead two) is the voltage between the right arm and the left leg.

In all combinations, right leg electrode does not come into play because the right leg RL electrode provides a reference potential (neutral (N) or ground.