The 10–20 system or International 10–20 system is an internationally recognized method to describe and apply the location of scalp electrodes in the context of an EEG exam, polysomnograph sleep study, or voluntary lab research. This method was developed to maintain standardized testing methods ensuring that a subject's study outcomes (clinical or research) could be compiled, reproduced, and effectively analyzed and compared using the scientific method. The system is based on the relationship between the location of an electrode and the underlying area of the brain, specifically the cerebral cortex. During sleep and wake cycles, the brain produces different, objectively recognized and distinguishable electrical patterns, which can be detected by electrodes on the skin. (These patterns might vary, and can be affected by multiple extrinsic factors, i.e. age, prescription drugs, somatic diagnoses, hx of neurologic insults/injury/trauma, and substance abuse) The "10" and "20" refer to the fact that the actual distances between adjacent electrodes are either 10% or 20% of the total front–back or right–left distance of the skull. For example, a measurement is taken across the top of the head, from the nasion to inion. Most other common measurements ('landmarking methods') start at one ear and end at the other, normally over the top of the head. Specific anatomical locations of the ear used include the tragus, the auricle and the mastoid. Electrode labeling 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). Note that there is no "central lobe"; due to their placement, and depending on the individual, the "C" electrodes can exhibit/represent EEG activity more typical of frontal, temporal, and some parietal-occipital activity, and are always utilized in polysomnography sleep studies for the purpose of determining stages of sleep. 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. These electrodes will not necessarily reflect or amplify lateral hemispheric cortical activity as they are placed over the corpus callosum, and do not represent either hemisphere adequately. "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. Note that the required number of