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This page describes products and general use; it is not medical advice.
The 10-20 system is the standard scheme for deciding where scalp electrodes go in a routine EEG (electroencephalogram). The American Clinical Neurophysiology Society (ACNS) guideline on electrode nomenclature describes it as a method proposed in 1958 that records EEG with 21 electrodes placed at relative distances of 10% or 20% between cranial landmarks. It is used by EEG technologists, neurophysiology clinic staff and students, and by buyers who must match caps, electrodes and recording equipment to the labels a lab uses.
What the 10-20 system is
The 10-20 system (also written “10 20 system”, “EEG 10-20” or “10-20 electrode system”) is a head-surface positioning method. Its name comes from the spacing: electrodes sit at 10% or 20% of the distance between fixed landmarks on the skull, so the layout scales to each head instead of using fixed centimetre distances. According to the ACNS guideline, the system was proposed by the International Federation of Societies for Electroencephalography and Clinical Neurophysiology in 1958 and has been the international standard for recording routine scalp EEG for clinical use. The same guideline notes that it has also served as a positioning reference for evoked and event-related potentials and for transcranial brain mapping methods.
How the layout is built: landmarks and 10%/20% spacing
Four landmarks anchor the layout: the nasion, the inion, and the left and right preauricular points. A sleep-research methods article in Current Protocols in Neuroscience (Campbell, 2009, NIH author manuscript) describes the inion as the highest point of the projection of the occipital bone at the lower rear of the skull, and the preauricular points as lying directly in front of the ear canal. Electrodes are then placed at 10% or 20% intervals of the distance between these landmarks.
This page does not give a head-measuring procedure. Measuring and applying electrodes on a person is done by trained technologists following the lab’s own protocol and the equipment instructions for use.
Electrode naming: letters, numbers and the T3/T7 change
In the nomenclature set out in the ACNS guideline, each electrode name encodes its position: all electrodes with the same letter or letters lie on the same coronal line, and, with a few exceptions (Fp1/Fp2 and O1/O2), electrodes on one front-to-back (sagittal) line share the same number. The one deviation from a strict combinatorial scheme is the “AF” (anterior frontal) row.
T3/T4 and T5/T6 versus T7/T8 and P7/P8
The older 10-20 labels T3/T4 and T5/T6 break that logic, so the 10-10 nomenclature replaces them with T7/T8 and P7/P8. This is a frequent source of confusion when reading cap labels or recordings: P7/P8 are posterior temporal positions, not parietal ones. The ACNS guideline states that P with a number of 6 or less is parietal, whereas P with a number of 7 or greater implies posterior temporal. The guideline also says (its recommendation 4) that it is an acceptable alternative to keep using T3/4 and T5/6, or to use both sets of terms, at present. Positions 10% below the frontotemporal electrodes are named F9/F10, T9/T10 and P9/P10.
10-20 vs 10-10 vs 10-5
The 10-20 layout has been extended. The table below states only what the ACNS guideline says about each.
| System | Electrode positions | Status per ACNS guideline |
|---|---|---|
| 10-20 | 21 electrodes | International standard for routine clinical scalp EEG |
| 10-10 | More than 70 positions described | Accepted as a standard by ACNS and the International Federation of Clinical Neurophysiology |
| 10-5 | Further extension for systems with many channels (the guideline cites 128 or 256 as examples) | Proposed, but not accepted by ACNS or the International Federation of Clinical Neurophysiology |
Source for the table: ACNS Guideline 2, Guidelines for Standard Electrode Position Nomenclature (checked 2026-10-30).
Equipment tied to the system
EEG electrodes applied to the skin fall under the FDA classification “Electrode, Cutaneous” (product code GXY, 21 CFR 882.1320), a Class 2 device. The regulation describes it as an electrode applied directly to a patient’s skin to record physiological signals, such as the electroencephalogram, or to apply electrical stimulation (openFDA and 21 CFR 882.1320, checked 2026-10-30).
For anyone matching equipment to a layout, the ACNS guideline makes a practical point: additional electrodes must be purchased and take technologist time, and most vendors of commercial EEG machines in the USA continued to provide headboxes with electrode positions and nomenclature limited to the 10-20 system (the guideline’s own statement). If a lab wants to use 10-10 positions, the recording hardware has to provide them. Related supplies, such as Ten20 conductive paste, ECG electrodes and sleep clinic supplies, are covered on separate CASRAI pages, and diagnostic and monitoring supplies are listed in the diagnostic, monitoring and imaging category.
Equipment criteria when choosing between 10-20 and 10-10
| Criterion | What the ACNS guideline says |
|---|---|
| Time and cost | The extra electrodes of the 10-10 system need more technologist time and effort, and the additional electrodes must be purchased |
| Recording hardware | Most vendors of commercial EEG machines in the USA continue to provide headboxes with electrode positions and nomenclature limited to the 10-20 system; headbox labels would have to change to show T7/T8 and P7/P8 |
| Label set (T3/T4, T5/T6 versus T7/T8, P7/P8) | Either set, or both, is an acceptable alternative at present |
Which electrode positions suit a given study is a clinical decision made by the clinician who orders and interprets it; this page does not advise on it.
Safe use and handling basics
Electrodes in the Class 2 “Electrode, Cutaneous” category are skin-contact devices, so the manufacturer’s instructions for use govern skin preparation, single-use or reuse status, cleaning and disposal. Follow those instructions and the facility’s infection-control policy rather than general advice.
Common mistakes
- Reading P7/P8 as parietal. Under the 10-10 nomenclature they are posterior temporal positions.
- Assuming a recording labeled T3/T4 is a different montage from one labeled T7/T8. The ACNS guideline treats the two label sets as acceptable alternatives for the same positions.
- Assuming 10-10 or 10-5 positions are available on any EEG system. Recording hardware must provide them.
- Treating the 10-5 extension as an accepted standard. The ACNS guideline says it has not been accepted by ACNS or the International Federation of Clinical Neurophysiology.
FAQ
What is the 10-20 system in EEG?
It is the standard method for placing 21 scalp electrodes at 10% or 20% distances between cranial landmarks for routine clinical EEG (ACNS guideline, checked 2026-10-30).
What does 10-20 stand for?
The numbers are the percentage spacing of electrodes along lines between landmarks: 10% or 20% of the measured distance.
Which landmarks does the 10-20 system use?
The nasion, the inion, and the left and right preauricular points (Campbell, 2009).
How many electrodes are in the 10-20 system?
21, according to the ACNS guideline. The 10-10 extension describes more than 70 positions.
What is the difference between the 10-20 and 10-10 systems?
10-10 adds electrodes in the anteroposterior, coronal and inferior planes and renames T3/T4 and T5/T6 as T7/T8 and P7/P8. See the table above.
Why do some labels say T3 and others T7?
The 10-10 nomenclature renamed T3/T4 as T7/T8 and T5/T6 as P7/P8 so that letters and numbers follow a consistent grid. The ACNS guideline says continuing to use the older labels, or both, is acceptable at present.
Sources
- American Clinical Neurophysiology Society, Guideline 2: Guidelines for Standard Electrode Position Nomenclature, checked 2026-10-30.
- Campbell IG, EEG Recording and Analysis for Sleep Research, Current Protocols in Neuroscience, 2009 (PMID 19802813), checked 2026-10-30.
- U.S. FDA product classification database (openFDA), “Electrode, Cutaneous”, product code GXY, and 21 CFR 882.1320, checked 2026-10-30.
