above the occipital point (at Oz). The Fp2, F8, T4, T6, and O2 electrodes are placed at intervals of 5%, 10%, 10%, 10%, 10%, and 5%, respectively, measured above the right ear, from front (Fpz) to back (Oz). The same is done for the odd-numbered electrodes on the left side, to complete the full circumference. Measurement methods for placement of the F3, F4, P3, and P4 points differ. If measured front-to-back (Fp1-F3-C3-P3-O1 and Fp2-F4-C4-P4-O2 montages), they can be 25% "up" from the front and back points (Fp1, Fp2, O1, and O2). If measured side-to-side (F7-F3-Fz-F4-F8 and T5-P3-Pz-P4-T6 montages), they can be 25% "up" from the side points (F7, F8, T5, and T6). If measured diagonally, from Nasion to Inion through the C3 and C4 points, they will be 20% in front of and behind the C3 and C4 points. Each of these measurement methods results in different nominal electrode placements. 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. EKG artifact can be reduced with post-filtering of signals, or by "jumping" (co-referencing) of A/M reference electrodes, if replacement of reference electrodes is not possible, ameliorative, or if other clinical considerations prevent otherwise good placement (such as congenital malformation, or post-surgical considerations such as Cochlear Implants). Higher-resolution systems When recording a more detailed EEG with more electrodes, extra electrodes are added using the 10% division, which fills in intermediate sites halfway between those of the existing 10–20 system. This new electrode-naming-system is more complicated giving rise to the Modified Combinatorial Nomenclature (MCN). This MCN system uses 1, 3, 5, 7, 9 for the left hemisphere which represents 10%, 20%, 30%, 40%, 50% of the inion-to-nasion distance respectively. The introduction of extra letter codes allows the naming of intermediate electrode sites. Note that these new letter codes do not necessarily refer to an area on the underlying cerebral cortex. The new letter codes of the MCN for intermediate electrode places are: AF – between Fp and F FC – between F and C FT – between F and T CP – between C and P TP – between T