Accurate eeg electrode placement is the foundation of every reliable brain recording, and it is the single skill that separates a clean, diagnostic study from a tracing full of artifact and confusion. When people ask what is an eeg test, the honest answer is that the test is only as good as the electrodes pressed against the scalp. An eeg test, short for electroencephalogram, records the tiny electrical signals produced by millions of neurons firing together, and those signals are measured in millionths of a volt, which is why precise positioning matters so much.

This guide explains the internationally recognized system used to position electrodes consistently across patients, clinics, and countries. Whether you are a student technologist studying for a board exam, a nurse preparing a patient, or a curious patient who wants to understand what is happening to your scalp, knowing how electrodes are mapped removes a great deal of anxiety. The standard approach is called the International 10-20 System, and once you understand the logic behind it, the rows of letters and numbers stop looking like alphabet soup and start making clear anatomical sense.

We will cover the bony landmarks technologists feel for, the measuring tape math that turns a head into a coordinate grid, and the naming convention that links each electrode to a region of the brain beneath it. We will also answer the practical questions patients search for most: how long is an eeg test, what does it cost, and whether the procedure carries any side effects. Spoiler — an EEG is painless and noninvasive, but proper preparation still improves your results dramatically and shortens the time you spend in the chair.

If you want to understand the diagnostic purpose behind all this careful placement, our companion article on what is a eeg test explains how recordings help neurologists identify epilepsy, classify seizure types, and localize the brain region where abnormal activity begins. Electrode placement and clinical interpretation are two sides of the same coin: you cannot localize a seizure focus accurately if the electrodes themselves were positioned by guesswork rather than measurement.

The 10-20 System was introduced in 1958 by Herbert Jasper and has been refined ever since, including expanded versions with 10-10 and 10-5 spacing for high-density recordings. Its genius is that it scales to any head size. A newborn, a petite adult, and a large-framed athlete all receive electrodes at proportionally identical positions, which means recordings can be compared across people and across time. This standardization is why a study done in Boston can be read by a neurologist in Seattle without ambiguity about where each waveform came from.

Throughout this article we will use the same vocabulary working technologists use every day. Terms like nasion, inion, preauricular point, Fpz, Cz, and Oz will become familiar landmarks rather than jargon. By the end you should be able to picture exactly where each of the standard 19 to 21 electrodes sits, understand why the spacing is described in percentages, and explain the entire procedure to a nervous patient in plain language. Let us start with the numbers that define a typical clinical study.

To map electrodes, a technologist first identifies four anatomical landmarks by touch. The nasion is the depression between your eyes at the top of the nose; the inion is the bony bump at the back of the skull where the neck muscles attach; and the two preauricular points are the small notches just in front of each ear canal. These landmarks are bony and consistent, which is what allows the percentage-based math to produce reliable, repeatable positions regardless of hair, head shape, or patient age.

With landmarks marked, the tech measures the total distance from nasion to inion over the top of the head, typically using a flexible cloth or paper tape. From that measurement, the midline electrodes are placed: Fpz at 10 percent up from the nasion, then Fz, Cz, Pz, and Oz at successive 20 percent intervals, with Oz sitting 10 percent above the inion. Cz, the vertex electrode, lands exactly at the halfway point and serves as the central reference around which much of the rest of the array is built.

The same logic governs the side-to-side measurement between the two preauricular points, which passes through Cz at the top of the head. The temporal and central chains are then derived from a circumferential measurement around the head's widest part. Each step uses the 10 and 20 percent rule, so the entire 21-electrode grid emerges from just three or four tape measurements and some quick mental arithmetic. Experienced technologists complete this measuring phase in only a few minutes.

The naming convention turns each position into a meaningful label. The letter identifies the brain region directly beneath the electrode, and the number identifies how far from the midline it sits. Fp stands for frontopolar, F for frontal, C for central, T for temporal, P for parietal, and O for occipital. Odd numbers — F3, C3, P3 — fall on the left hemisphere; even numbers — F4, C4, P4 — fall on the right; and the letter z, standing for zero, marks every midline position such as Fz, Cz, and Pz.

Once positions are marked with a wax pencil or skin-safe marker, the scalp is gently abraded with a mild prep gel and the electrodes are attached using conductive paste or collodion adhesive. Each disc-shaped electrode is roughly a centimeter wide and made of silver/silver-chloride or gold. The conductive medium bridges the gap between metal and skin so the faint electrical signals can travel to the amplifier. Hair is parted at each site so the metal makes direct contact with the scalp rather than resting on strands.

If you are curious about the people who perform this skilled work and what they earn, our guide on how long is an eeg test also covers the technologist career path, certification, and regional pay differences. Mastering precise placement is exactly the competency that distinguishes a registered EEG technologist (R. EEG T.) from an untrained assistant, and it is heavily tested on the ABRET credentialing examinations that govern the profession in the United States.

Finally, every electrode connects to a head box, and the technologist labels and verifies each channel before recording begins. A quick visual map — the montage — tells the reading neurologist which physical electrode produced which waveform. Without standardized placement, those waveforms would be impossible to compare or localize, which is why this measuring discipline remains the bedrock of the entire test.

Once electrodes are physically in place, the battle shifts to signal quality, and the central concept here is impedance — the resistance the skin offers to the tiny electrical signals trying to reach the amplifier. High impedance lets electrical noise from lights, equipment, and the patient's own body contaminate the recording. The goal in a clinical lab is to keep each electrode below 5,000 ohms (5 kilohms) and to keep impedances roughly balanced across channels so that no single electrode introduces disproportionate artifact into the tracing.

Lowering impedance starts with skin preparation. After marking each position, the technologist gently abrades the top layer of dead skin cells with a mild gritty prep gel applied on a cotton swab or blunt wooden stick. This is not painful — it feels like a light scrub — but it dramatically improves the electrical bridge between scalp and metal. Oils, hairspray, and dry skin all raise impedance, which is exactly why patients are told to arrive with clean, product-free hair on the day of the test.

Artifact is any signal in the recording that does not come from the brain, and recognizing it is a core skill. Common offenders include eye blinks, which produce large slow deflections in the frontal electrodes; muscle tension in the jaw and neck, which adds fast spiky noise; and the steady 60-hertz hum from electrical wiring in North American buildings. Sweat can create slow rolling waves, while a loose electrode produces sudden pops. Technologists learn to spot each signature and address its physical cause rather than simply filtering it away.

Movement is the enemy of a clean tracing. Even small head adjustments can dislodge an electrode or stretch a wire, so patients are asked to stay still and relaxed. For pediatric or agitated patients, technologists use extra collodion adhesive and sometimes a head wrap to keep electrodes secure. The recording software displays real-time impedance values, often color-coded green, yellow, and red, allowing the tech to re-prep any problem site before the diagnostic portion of the study begins.

Filters help but cannot rescue a poorly prepared recording. A low-frequency filter removes slow sweat and movement drift, a high-frequency filter tames muscle noise, and a notch filter specifically targets the 60-hertz line frequency. However, overusing filters can hide genuine brain activity, so reading neurologists prefer to see minimally filtered data whenever the underlying electrode contact is good. This is yet another reason careful placement and prep matter more than any software trick applied after the fact.

Visual references make these concepts far easier to learn, and our gallery covering eeg test side effects also displays annotated tracings that show exactly what blink artifact, muscle artifact, and clean background rhythm look like side by side. Comparing a contaminated channel to a well-prepared one is the fastest way for students to internalize why the unglamorous work of skin prep determines whether a study is diagnostic or worthless.

Documentation closes the loop. Before recording, the technologist confirms the montage, notes any electrodes that could not reach target impedance, and records the patient's level of alertness. These notes travel with the study so the interpreting physician knows whether an unusual finding reflects genuine brain activity or a technical limitation. Good technologists treat their notes as seriously as their electrode placement, because together they tell the complete story of the recording.

While the standard 19-channel 10-20 array covers most routine clinical needs, real-world recordings often call for specialized montages and supplemental electrodes. A montage is simply the organized way channels are displayed and compared, and the two main families are bipolar and referential. In a bipolar montage, each channel shows the voltage difference between two adjacent scalp electrodes, which is excellent for localizing focal abnormalities. In a referential montage, every electrode is compared against a common reference, which preserves the true amplitude and shape of widespread activity.

Temporal lobe epilepsy is so common that technologists frequently add electrodes to better capture the temporal region, where many seizures originate. Anterior temporal electrodes (often labeled T1 and T2) and the so-called true temporal positions improve detection of discharges that the standard array might miss or underrepresent. Sphenoidal electrodes, inserted near the base of the skull, go even further for select epilepsy-surgery candidates, though these are specialized and placed by physicians rather than routine techs.

High-density EEG expands the standard grid using the 10-10 and 10-5 systems, packing 64, 128, or 256 electrodes onto the scalp. These dense arrays dramatically improve spatial resolution and are essential for source localization, brain-computer interfaces, and research mapping. They are usually applied with an elastic electrode cap rather than individual discs, which speeds setup but still requires the same proportional landmark logic to position the cap correctly on each head.

Neonatal and pediatric EEG uses a reduced electrode set because infant heads are small and the scalp is delicate. A neonatal montage may use only nine or so electrodes positioned with the same percentage rules scaled to a tiny head. Technologists working with children rely heavily on patience, distraction, and speed, since a wiggling toddler offers a narrow window to apply electrodes before cooperation runs out. Caps and quick-application pastes are favored in these settings.

Ambulatory and long-term monitoring montages prioritize durability because electrodes must survive 24 hours to several days of normal activity. Collodion adhesive and protective wraps keep contacts in place while patients eat, sleep, and move at home. If you want a deeper look at extended recordings, our overview of the 24-hour study explains how these montages capture infrequent events that a brief office test would never witness, dramatically raising the diagnostic yield for elusive symptoms.

Cost is also tied to montage complexity, and understanding the breakdown helps patients plan. Our article on eeg test cost explains how routine, ambulatory, and video-EEG studies are priced differently because longer monitoring requires more technologist time, more equipment, and often a hospital stay. Knowing which study your neurologist ordered lets you anticipate both the time commitment and the likely bill before you arrive.

Across every variation, the underlying philosophy never changes: position electrodes proportionally using bony landmarks, achieve low and balanced impedance, and document everything. Whether a tech places nine electrodes on a newborn or 256 on a research volunteer, the same 10-20 logic governs the geometry. Mastering that core system is what allows a technologist to adapt confidently to any clinical situation that walks through the door.

Whether you are a patient or a student, a little preparation makes the EEG experience smoother and the results more reliable. For patients, the most important step is hair hygiene: wash thoroughly the night before, skip all styling products, and arrive with dry hair. Eat normally unless your doctor orders a sleep-deprived study, and ask whether to continue your usual medications — neurologists often want you on your normal regimen so the recording reflects your everyday brain activity rather than an artificially altered state.

Bring a list of your medications, a comfortable change of plan for messy hair afterward, and a friend or family member if you will be sleep-deprived and unable to drive safely. If your study involves photic stimulation with a flashing strobe, tell the technologist about any history of light-triggered seizures so they can monitor you closely. None of these steps is complicated, but each one nudges the recording toward being clean, diagnostic, and free of avoidable repeats.

For aspiring technologists, the path to mastery is repetition under supervision. Measure dozens of heads until the nasion-to-inion math becomes automatic, and practice palpating landmarks on patients of every age and head shape. Speed without accuracy is worthless, so build precision first and let speed follow naturally. Strong candidates also memorize the naming convention cold, because reading neurologists expect flawless channel labeling and will quickly lose confidence in a tech who mislabels left and right.

Skin preparation deserves dedicated practice because it is where most quality problems originate. Learn how much abrasion is enough to lower impedance without irritating the scalp, and develop a consistent routine for applying paste so electrodes adhere firmly the first time. Keep a mental checklist running during setup: landmark, measure, mark, prep, apply, verify. Technologists who follow the same sequence every single time make fewer errors and finish faster than those who improvise their order on each patient.

Study the artifacts relentlessly, because recognizing them in real time lets you fix problems before the diagnostic recording begins rather than apologizing for them afterward. Know what a sweat artifact looks like versus a slow-wave abnormality, and know how to distinguish a 60-hertz buzz from genuine fast activity. The best technologists develop an instinct for when a tracing looks wrong, and they trust that instinct enough to stop, re-prep an electrode, and protect the integrity of the study.

Certification anchors the profession in the United States, and the ABRET-administered credentials reward exactly this blend of hands-on skill and theoretical knowledge. Practice questions covering placement, montages, activation procedures, and abnormal patterns are among the most effective ways to prepare, because they force you to apply the 10-20 logic to realistic scenarios. Pair question banks with supervised lab time, and you will walk into both the exam and the clinic with genuine confidence rather than memorized fragments.

Finally, never lose sight of the patient in front of you. A calm explanation, a warm room, and a few minutes of reassurance reduce muscle tension and movement artifact more effectively than any filter. Patients who understand that the test is painless, that the electrodes only listen and never shock, and that the messy paste washes right out tend to relax, hold still, and produce beautiful recordings. Technical excellence and bedside kindness are not separate skills — together they define a great EEG technologist.