EEG electrodes, and their careful, measured placement, increases with each clinical requirement and modality. Even-numbered electrodes (2,4,6,8) refer to electrode placement on the right side of the head, whereas odd numbers (1,3,5,7) refer to those on the left; this applies to both EEG and EOG (electrooculogram measurements of eyes) electrodes, as well as ECG (electrocardiography measurements of the heart) electrode placement. Chin, or EMG (electromyogram) electrodes are more commonly just referred to with "right," "left," and "reference," or "common," as there are usually only three placed, and they can be differentially referenced from the EEG and EOG reference sites. The "A" (sometimes referred to as "M" for mastoid process) refers to the prominent bone process usually found just behind the outer ear (less prominent in children and some adults). In basic polysomnography, F3, F4, Fz, Cz, C3, C4, O1, O2, A1, A2 (M1, M2), are used. Cz and Fz are 'ground' or 'common' reference points for all EEG and EOG electrodes, and A1-A2 are used for contralateral referencing of all EEG electrodes. This EEG montage may be extended to utilize T3-T4, P3-P4, as well as others, if an extended or "seizure montage" is called for. Measurement Specific anatomical landmarks are used for the essential measuring and positioning of the EEG electrodes. These are found with a tape measure, and often marked with a grease pencil, or "China marker." Nasion to Inion: the nasion is the distinctly depressed area between the eyes, just above the bridge of the nose, and the inion, is the crest point of back of the skull, often indicated by a bump (the prominent occipital ridge, can usually be located with mild palpation). Marks for the Z electrodes are made between these points along the midline, at intervals of 10%, 20%, 20%, 20%, 20% and 10%. Preauricular to preauricular (or tragus to tragus: the tragus refers to the small portion of cartilage projecting anteriorly to the pinna). The preauricular point is in front of each ear, and can be more easily located with mild palpation, and if necessary, requesting patient to open mouth slightly. The T3, C3, Cz, C4, and T4 electrodes are placed at marks made at intervals of 10%, 20%, 20%, 20%, 20% and 10%, respectively, measured across the top of the head. Skull circumference is measured just above the ears (T3 and T4), just above the bridge of the nose (at Fpz), and just