10–20 system (EEG)

Across all phases of consciousness, brains produce different, objectively recognizable and distinguishable electrical patterns, which can be detected by electrodes on the skin.

These patterns vary, and are affected by multiple extrinsic factors, including age, prescription drugs, somatic diagnoses, history of neurologic insults/injury/trauma, and substance abuse.

Each electrode placement site has a letter to identify the lobe, or area of the brain it is reading from: pre-frontal (Fp), frontal (F), temporal (T), parietal (P), occipital (O), and central (C).

There are also (Z) sites: A "Z" (zero) refers to an electrode placed on the midline sagittal plane of the skull, (FpZ, Fz, Cz, Oz) and is present mostly for reference/measurement points.

"Z" electrodes are often utilized as 'grounds' or 'references,' especially in polysomnography sleep studies, and diagnostic/clinical EEG montages meant to represent/diagnose epileptiform seizure activity, or possible clinical brain death.

When placing the A (or M) electrodes, palpation is often necessary to determine the most pronounced point of the mastoid process behind either ear; failure to do so, and to place the reference electrodes too low (posterior to the ear pinna, proximal to the throat) may result in "EKG artifact" in the EEGs and EOGs, due to artifact from the carotid arteries.