The Math and Anatomy of the 10–20 System

Do not be intimidated by the word "math." The 10–20 System uses simple percentages, not calculus, and once you have measured a few heads it becomes automatic. But do take it seriously, because this is the foundation the rest of the study rests on. Get the placement right and everything downstream — the signals, the staging, the scoring — has a fighting chance. Get it wrong and you can quietly corrupt the entire recording.

In this chapter you will learn to:

• Explain what the 10–20 System is and why accurate placement matters.

• Locate the four skull landmarks and find Cz.

• Measure front-to-back and side-to-side and use the 10% and 20% percentages.

• Identify the standard PSG electrode locations, including the AASM-recommended derivations.

• Recognize how bad placement corrupts scoring, and avoid the common measurement mistakes.

• Handle hair, scalps, patient comfort, and documentation professionally.

What the 10–20 System is

The International 10–20 System is the standardized method for placing EEG electrodes on the scalp so that the same location means the same thing on every patient, in every lab, anywhere in the world. Before such a system existed, one technologist's "frontal" electrode might sit somewhere quite different from another's, making recordings impossible to compare. The 10–20 System solved that by tying every electrode position to fixed anatomical landmarks and to proportional distances between them.

The name explains the method: electrode positions are spaced at intervals of 10% or 20% of the distance between specific skull landmarks. The genius of using percentages rather than fixed centimeter measurements is that it automatically adapts to any head. A large adult skull and a small child's skull have very different absolute dimensions, but 20% of the distance between two landmarks lands on the same functional spot on both. The system scales itself. That is why you will always be measuring and multiplying, never just placing electrodes "about here."

Why accurate placement matters

It is tempting, especially when a patient is tired and you are behind schedule, to eyeball placement and move on. Resist that temptation completely, because electrode position directly determines what signal you record. The brain's electrical activity varies by region — frontal, central, and occipital areas each contribute different, characteristic patterns that scorers rely on to identify sleep stages. An electrode placed too far forward, too low, or off-center picks up the wrong mix of activity.

The consequences are not cosmetic. As you will see in Part III, staging depends on recognizing specific waveforms that are best seen at specific locations. Misplaced electrodes can attenuate or distort those waveforms, making stages ambiguous, mimicking or hiding events, and forcing the scorer to guess. A physician then reads a study built on corrupted data. Accurate placement is, in a real sense, an act of patient care: it is how you make sure the answer the study produces is actually true.

The four skull landmarks

Every measurement starts from four bony landmarks you can feel with your fingers. Learn to find these confidently, because everything else is measured from them.

• Nasion — the small dip at the top of the bridge of the nose, right between the eyes where the nose meets the forehead. This is your front anchor.

• Inion — the bony bump at the back of the skull, at the base of the head along the midline. Tilt your own head and run a finger up the back of your neck until you feel the ridge; the bump is the inion. This is your back anchor.

• Left preauricular point — the small notch just in front of the ear canal on the left side, where you feel a dip if you open and close your jaw. This is your left-side anchor.

• Right preauricular point — the same notch in front of the right ear. Your right-side anchor.

The nasion and inion define the front-to-back midline. The two preauricular points define the side-to-side line. Where those two lines cross is the top of the head — and that intersection is how you find your first and most important electrode reference, Cz.

Figure — Two simple head outlines, one side view and one front-top view, with labeled dots marking the nasion between the eyes, the inion at the back of the skull, and the preauricular points just in front of each ear, plus dashed front-to-back and side-to-side reference lines.

Figure 5.1 The four bony landmarks that anchor every 10–20 measurement: the nasion in front, the inion at the back, and the left and right preauricular points at the ears.

Finding Cz: the anchor of the whole montage

Cz is the vertex — the point at the very top of the head — and it is the reference point most of your placements build from. Finding it is a two-measurement job.

First, measure the front-to-back distance from nasion to inion with your tape measure, following the midline over the top of the head. Cz sits at 50% of that distance — exactly halfway from nasion to inion.

Second, measure the side-to-side distance from the left preauricular point to the right preauricular point, up and over the top of the head. Cz also sits at 50% of that distance — exactly halfway between the ears.

Cz is where those two 50% marks meet. Mark the halfway point of the front-back line, mark the halfway point of the side-to-side line, and the single spot where both are true is Cz. If your two marks do not cross at the same point, you have a measurement error — re-measure. Getting Cz right is worth the extra care because every other placement inherits its accuracy (or its error) from this step.

Figure — Top-of-head diagram showing a nasion-to-inion line marked at its 50% midpoint and an ear-to-ear line marked at its 50% midpoint, with Cz highlighted where the two midpoints intersect.

Figure 5.2 Locating Cz at the intersection of the 50% front-to-back mark (nasion–inion) and the 50% side-to-side mark (ear–ear).

Measuring front-to-back and side-to-side with percentages

Once you understand Cz, the logic of the whole system clicks into place. Along the front-to-back midline, positions are laid out as percentages of the nasion–inion distance. Working back from the nasion, the midline points fall at 10%, then in 20% steps. The frontal pole region (Fp) sits near the front at 10% up from the nasion; the central line (through Cz) sits at 50%; the occipital region (O) sits near the back, 10% up from the inion. The same proportional logic governs the side-to-side and the intermediate measurements: you measure the total distance between two landmarks, then place electrodes at the defined 10% and 20% fractions of it.

The essential mental model is this: you never guess a distance; you measure a total and take a percentage of it. Because the percentages are fixed and the landmarks are fixed, the only variable is your care in measuring. That is genuinely good news — it means accuracy is a skill you can master through practice, not a matter of talent or guesswork.

Standard PSG electrode locations

A full diagnostic EEG can use many electrodes, but a standard polysomnogram uses a focused subset. The current AASM-recommended setup samples three brain regions on each side — frontal, central, and occipital — plus the mastoids behind the ears as references. Here are the locations you must know cold, with what each represents.

• Fpz — a frontal-pole midline position near the front of the head (10% up from the nasion on the midline). Often used as a reference or ground position in PSG setups.

• F3 — left frontal.

• F4 — right frontal.

• C3 — left central.

• C4 — right central.

• Cz — the vertex/midline central reference you located above.

• O1 — left occipital (back of the head).

• O2 — right occipital.

• M1 — left mastoid, the bony bump behind the left ear (a reference electrode).

• M2 — right mastoid, behind the right ear (a reference electrode).

Two naming conventions make these easy to decode. The letter tells you the region: Fp = frontal pole, F = frontal, C = central, O = occipital, M = mastoid. The number tells you the side: odd numbers are on the left (F3, C3, O1, M1), even numbers are on the right (F4, C4, O2, M2), and z ("zero") means the midline (Fpz, Cz). Learn "odd = left, even = right, z = midline" and half the map is memorized already.

The AASM-recommended derivations

In PSG, we do not read a single electrode in isolation; we read the difference between two electrodes, called a derivation. The current AASM-recommended EEG derivations for staging are F4–M1, C4–M1, and O2–M1 — that is, the right frontal, right central, and right occipital electrodes each referenced to the left mastoid. A backup set (F3–M2, C3–M2, O1–M2, the left-side electrodes referenced to the right mastoid) is placed so that if a primary electrode fails during the night, the scorer can switch to the mirror-image channel without losing the study. This is exactly why you place electrodes on both sides even though only one set is "primary": the backups protect the night. (Requirements and recommended derivations are periodically updated — verify the current requirement directly with the AASM before relying on it.)

Figure — Top-of-head circle showing labeled positions Fpz at front midline, F3 and F4 frontal, C3, Cz, and C4 central, O1 and O2 occipital at the back, and M1 and M2 at the mastoids behind the ears, with left positions odd-numbered and right positions even-numbered.

Figure 5.3 A conceptual top-of-head map of the standard PSG electrode positions, showing the odd-left / even-right / z-midline naming logic and the mastoid references behind the ears.

How incorrect placement corrupts scoring

It is worth making the link between this chapter and Part III explicit. Sleep stages are identified largely by waveforms that are strongest in particular regions: certain slow activity and specific graphoelements are best captured frontally, while some defining rhythms of drowsiness and wake are best seen occipitally. If your frontal electrodes are misplaced, the waveforms that define deep sleep may look smaller than they are, and a scorer may under-call that stage. If your occipital electrodes are off, the rhythm used to identify quiet wakefulness may be weak or absent, blurring the wake-to-sleep transition. Reference errors at the mastoids can distort every derivation that depends on them.

The pattern is always the same: a small placement error at setup becomes a large interpretive error at scoring, and neither the scorer nor the physician may realize the data was compromised. That is why "close enough" is not acceptable here. Your careful tape measure at 9 p.m. protects the truth of a diagnosis made days later.

A step-by-step measurement walkthrough

Here is the whole process in order. Practice it until it flows.

1. Seat the patient comfortably and explain what you are about to do (see the patient-comfort section below).

1. Locate the four landmarks — nasion, inion, left and right preauricular points — with your fingers, confirming each.

1. Measure nasion to inion over the midline; note the total. Mark 50% for the Cz front-back line.

1. Measure left to right preauricular over the top; note the total. Mark 50% for the Cz side-to-side line.

1. Set Cz where the two 50% marks intersect; re-measure if they don't agree.

1. Lay out the midline points as percentages of the nasion–inion distance (Fp region near the front, occipital region near the back).

1. Place the paired positions (F3/F4, C3/C4, O1/O2) at their defined percentages, keeping left/right symmetry.

1. Place the mastoid references (M1, M2) behind each ear.

1. Prep the skin and apply each electrode, then verify signal quality and impedances (covered in Chapter 6).

1. Document the placement, noting any deviations or problems.

Practice problems

Work these to make the percentages automatic. (Answers follow.)

1. A patient's nasion-to-inion distance measures 36 cm. How far from the nasion, in centimeters, is the 50% mark where Cz's front-back line sits?

1. A patient's ear-to-ear distance measures 34 cm. How far from the left preauricular point is the 50% mark for Cz?

1. If a nasion-to-inion distance is 38 cm, how many centimeters is 10% of it (the spacing used to step in from the front and back anchors)?

1. Which side of the head do the electrodes F3, C3, and O1 sit on, and how do you know from the numbers?

1. A trainee measures nasion-to-inion and gets a Cz front-back mark that does not line up with the Cz side-to-side mark. What has gone wrong, and what should they do?

Answers: (1) 18 cm. (2) 17 cm. (3) 3.8 cm. (4) The left side — odd numbers are left. (5) A measurement error somewhere; re-measure both distances and re-mark, since Cz must satisfy both 50% lines at once.

Patients, hair, scalps, and comfort

Landmarking is intimate. You are touching a patient's head and face, parting their hair, and marking their scalp, and doing it with respect is part of the professionalism of the job.

Explain the process to the patient before and during. A simple, plain-language narration — "I'm going to measure your head so the sensors go in exactly the right spots; you'll feel me pressing with a tape measure and marking a few points" — removes the strangeness and builds trust. Tell them what they will feel before they feel it.

Handle hair, wigs, and hairstyles thoughtfully. Thick, braided, curly, or treated hair, extensions, and wigs all affect access to the scalp. Ask the patient about their hair, be gentle parting it, and work with their hairstyle rather than against it. If a wig or hairpiece must be removed, handle it and the request with dignity and privacy. Never make a patient feel their hair is an inconvenience.

Be gentle with sensitive scalps. Some patients have tender skin, skin conditions, or soreness. Adjust your pressure, check in about comfort, and be careful with prep. Comfort is not a nicety; an uncomfortable patient sleeps poorly and gives you a poorer study.

Protect dignity throughout. Explain, ask permission, work efficiently, and keep the patient informed. The way you handle these small human moments often shapes how the whole night goes.

Documenting placement problems

Not every setup is textbook. A patient may have anatomy that shifts a landmark, a wound or device that blocks a position, or hair that prevents ideal contact. When you must deviate from standard placement, document it clearly: note what you could not place normally, where you placed it instead, and why. Accurate documentation lets the scorer and physician interpret the data correctly instead of being misled by an unexplained oddity, and it protects both the patient and you. Say "see Chapter 8" for how this fits into overall study documentation. The rule is simple: if the setup differs from standard, the record should say so.

Figure 5.4 Frequent 10–20 measurement errors, what they cause, and how to prevent them.

A checklist for electrode landmarking

• Patient seated comfortably and told what to expect.

• All four landmarks located and confirmed by touch.

• Nasion–inion measured; 50% marked.

• Ear-to-ear measured; 50% marked.

• Cz set at the true intersection (re-measured if the marks disagreed).

• Paired positions placed symmetrically at correct percentages.

• Mastoid references (M1, M2) placed.

• Hair and scalp handled gently; patient comfortable.

• Any deviation from standard placement documented.

Clinical Takeaways

• The 10–20 System uses percentages of landmark distances so the same position means the same thing on any head.

• Find Cz first; it anchors the whole montage, and its accuracy (or error) propagates to every other position.

• "Odd = left, even = right, z = midline" decodes every electrode name.

• The AASM-recommended derivations are F4–M1, C4–M1, and O2–M1, with F3–M2, C3–M2, O1–M2 as backups — which is why you place both sides.

• A small placement error at setup becomes a large scoring error later; "close enough" is not acceptable.

• Handling hair, scalp, comfort, and documentation respectfully is part of the technical skill, not separate from it.

Study Questions

1. Why does the 10–20 System use percentages instead of fixed centimeter distances?

1. Name the four skull landmarks and what each one anchors.

1. Describe, step by step, how you locate Cz.

1. Decode these names by region and side: F3, C4, O1, M2, Cz.

1. What are the AASM-recommended EEG derivations, and why do you place electrodes on both sides of the head?

1. Give two examples of how incorrect placement can distort scoring.

1. What should you do, and document, when a patient's anatomy or hair forces you to deviate from standard placement?

Lab Reality Check

The 10–20 System is one of those skills that feels awkward and slow for your first dozen patients and then, almost without your noticing, becomes second nature — you'll find your fingers finding the inion automatically while you chat with the patient. But watch out for the trap that catches tired technologists: the temptation to eyeball it "just this once" at the end of a long week. The veterans who produce consistently clean, easily-scored studies are the ones who still measure every time, even at 10 p.m. on their fourth night in a row. You'll also learn quickly that real heads are not textbook diagrams — there are cowlicks, scars, thick braids, surgical sites, and every kind of hair, and the good technologist adapts with patience and respect rather than force. Finally, remember that the scorer and physician downstream can only trust what you place. Your tape measure is the first link in the chain of a correct diagnosis. Treat it that way, and document honestly when a head won't cooperate.

Explain Like I Am 10

Imagine you need to put stickers on someone's head in the exact same spots every single time, even though everybody's head is a different size. How would you do it fairly? You wouldn't use a ruler that says "3 inches from the nose," because 3 inches lands in a different place on a big head than a small head. Instead, you use fractions of the whole head. That clever idea is called the 10–20 System, because the stickers go at spots that are one-tenth (10%) or one-fifth (20%) of the way between certain bumps on the skull. Fractions stretch and shrink to fit any head — pretty smart!

First you find four "bumps and dips" to measure from: the little dip between the eyes (nasion), the bump on the back of the head (inion), and the little notches right in front of each ear (preauricular points). Then you find the tip-top of the head, called Cz, by measuring halfway from front to back and halfway from ear to ear — Cz is where those two halfway marks cross, like the center of a treasure map. Every other sticker gets measured out from there.

The stickers have secret-code names. The letter tells you where (F = forehead/frontal, C = center, O = back/occipital), and the number tells you which side: odd numbers are on the left, even numbers are on the right, and a "z" means right down the middle. So F3 is "frontal, left" and O2 is "back of head, right." If you put a sticker in the wrong spot, the brain signals come out looking weird, and the person reading them later might get the wrong answer — so measuring carefully really matters. And because you're touching someone's head and hair, you do it gently and kindly, and you explain what you're doing so they're not scared.

Remember This

• The 10–20 System places EEG stickers using fractions of the head, so it fits everybody.

• You measure from four spots: nasion, inion, and the two ear notches.

• Cz is the top of the head, where the halfway marks cross — find it first.

• Sticker names: letter = where, number = side (odd = left, even = right, z = middle).

• Wrong placement = wrong signals, so measure carefully and treat the patient's head gently.

Quick Review Questions

1. Why do we use fractions of the head instead of a plain ruler measurement?

1. Where is Cz, and how do you find it?

1. Is O1 on the left side or the right side? How do you know?

1. What could happen if you put an electrode in the wrong spot?

1. Name one way to be kind and respectful while measuring someone's head.

Sources (verified for this chapter):

• AASM recommended EEG electrode placement (F4-M1, C4-M1, O2-M1) — summary via peer-reviewed literature: The 2007 AASM Recommendations for EEG Electrode Placement in Polysomnography (PMC)

• 10–20 System overview (percentages and landmarks): 10–20 system (EEG), reference overview)

• Always confirm the current recommended derivations and montage against the latest AASM Manual for the Scoring of Sleep and Associated Events.

Common Mistakes

• Eyeballing placement to save time. It corrupts the data you are being paid to capture accurately.

• Getting Cz slightly wrong. Because everything references it, the error spreads across the montage.

• Confusing odd and even (left and right). Double-check the numbering every time.

• Forgetting the backup electrodes. Without the mirror set, a single failed electrode can cost you the night.

• Fighting the patient's hair instead of working with it. It causes poor contact and an uncomfortable patient.

• Deviating from standard placement without documenting it. It leaves the scorer to misread compromised data.