In Chapter 7 of Intermediate Physics for Medicine and Biology, Russ Hobbie and I discuss the 10–20 system of electrodes on the scalp used to record the electroencephalogram.

Much can be learned about the brain by measuring the electric potential on the scalp surface. Such data are called the electroencephalogram (EEG)… Typically, the EEG is measured from 21 electrodes attached to the scalp according to the 10–20 system (Fig. 7.34).

Why is this placement of electrodes called the “10–20” system? Consider the path starting at the nasion (between the eyes, just above the bridge of the nose), passing over the top of the head, and ending at the inion (a small protuberance at the lower back of the skull). This path is shown as the vertical dashed line in the top view of the head in Fig. 7.34. Five electrodes (Oz, Pz, Cz, Fz, and Fpz) are placed 10, 20, 20, 20, 20, and 10% of the distance along the path. All those 10s and 20s give rise to the name “10–20 system.” The electrodes Oz and Fpz aren’t part of the 10–20 system, so they’re not shown in Fig. 7.34, but their positions are used to properly place the other electrodes.

Now, consider the path starting just behind the left ear (auricle, A1, sometimes known as the mastoid), passing over the top of the head through Cz, and ending just behind the right ear (A2); the horizontal dashed line in Fig. 7.34. Five electrodes (T3, C3, Cz, C4, and T4) are placed 10, 20, 20, 20, 20, and 10% of the distance along the path.

Next, examine the dashed circle in Fig. 7.34, which represents a circumference of the head through Oz, T3, Fpz, and T4. Ten electrodes (O1, T5, T3, F7, Fp1, Fp2, F8, T4, T6, and O2) are equally spaced along this circumference, each 10% of the way around the circle.

Finally, consider a great circle path passing from Fp1 through C3 to O1. The electrode F3 is halfway between Fp1 and C3. Similar reasoning gets you the positions of P3, F4 and P4.

How do these electrodes get their funny names? The first letter indicates the region of the brain: F for frontal (front), T for temporal (side, named for your temples), P for parietal (center-back), O for occipital (lower back), Fp for pre-frontal, and C for central. A subscript z means along the midline. Even numbers are used for the right of the head, and odd numbers for the left.

The 10–20 system was proposed by a committee of the International Federation of Clinical Neurophysiology, in order to standardize EEG recordings among different laboratories.

Originally published at

Professor of Physics at Oakland University and coauthor of the textbook Intermediate Physics for Medicine and Biology.