The International 10-20 System and Sleep Montage Landmarks

1. Landmarks and cranial measurements

The system is anchored by four landmarks: the nasion (indentation at the bridge of the nose), the inion (bony prominence at the back of the skull), and the left and right preauricular points (the depressions just in front of each ear's tragus). Cranial measurements are taken along defined paths: anterior-posterior (nasion to inion over the vertex), transverse (preauricular point to preauricular point through the vertex), and circumferential (around the head). These define the grid on which proportional placement is based.

2. Proportional placement and electrode labeling

Proportional placement locates sites at 10% and 20% intervals of the measured distances rather than fixed physical distances. This is the standardized-placement rationale: it makes placement reproducible across head sizes, technologists, and laboratories, allowing valid comparison over time and across labs, and it aligns sites with known brain regions. Electrode labeling follows a code: the letter indicates the region (F = frontal, C = central, O = occipital, T = temporal, P = parietal), and the number or letter indicates side — odd for left, even for right, "z" for midline. Thus F3/F4 are left/right frontal, C3/C4 left/right central, and O1/O2 left/right occipital.

3. The modified 10-20 sleep montage and derivations

The modified 10-20 sleep montage is a reduced electrode set sufficient for staging and event recognition, commonly F3, F4, C3, C4, O1, O2, and the references M1 and M2 (left and right mastoid). A1/A2 terminology context: older systems called these ear references A1 (left) and A2 (right); current AASM terminology uses M1/M2, but technologists should recognize both in legacy literature and equipment. A referential montage records each electrode against a common reference (e.g., F4 against contralateral M1), preserving local amplitude; a bipolar montage records the difference between two active electrodes, helping localize activity. Sleep staging relies primarily on referential derivations per current guidance, but the exact montage can vary by institution.

How it works

1. The four landmarks anchor the measurement grid.
2. Cranial measurements (anterior-posterior, transverse, circumferential) define the distances.
3. Sites are located proportionally at 10%/20% intervals, fitting any head size.
4. Labels encode region (letter) and side (odd left, even right, "z" midline).
5. The modified 10-20 montage selects the staging sites (F3/F4, C3/C4, O1/O2, M1/M2).
6. Derivation (referential vs. bipolar) determines how each channel is displayed, and placement is documented.

Common confusions

Memory aids

Landmarks: "N-I-P-P" — Nasion, Inion, Preauricular (left/right). Sleep-montage electrodes: "F-C-O-M" — Frontal, Central, Occipital, Mastoid. Labeling: "odd left, even right, z is midline."

Quick review

Topic Recap

• The 10-20 system locates electrodes by proportional 10%/20% intervals.
• Four landmarks anchor it: nasion, inion, left/right preauricular points.
• Cranial measurements are anterior-posterior, transverse, and circumferential.
• The modified sleep montage commonly uses F3, F4, C3, C4, O1, O2, M1, M2.
• Referential vs. bipolar derivations change how voltage is displayed.
• Documentation and awareness of institutional montage variation are essential.

Knowledge Check

1. What are the four landmarks anchoring the 10-20 system?
2. Why is placement proportional rather than based on fixed distances?
3. Which electrodes typically make up the modified 10-20 sleep montage?
4. How does a referential montage differ from a bipolar montage?
5. What do M1 and M2 stand for, and how do they relate to A1/A2?

Answers and Rationales

1. Nasion, inion, and the left/right preauricular points. Why: They are the fixed anchors for all measurements.
2. So sites fall at the same relative brain locations on any head size, keeping recordings comparable. Why: Fixed distances would shift positions on different-sized heads.
3. Typically F3, F4, C3, C4, O1, O2, and mastoid references M1, M2. Why: This reduced set is sufficient for staging.
4. Referential records each electrode against a common reference; bipolar records the difference between two active electrodes. Why: Referential preserves local amplitude; bipolar localizes activity.
5. M1/M2 are the current left/right mastoid references; A1/A2 are the older ear-reference labels for the same role. Why: Recognizing both avoids legacy/current confusion.