04/02/2026
To determine EEG electrode placement for a polysomno-graphic recording according to AASM criteria, it is necessary to measure the patient’s head and mark the F3/F4, C3/C4, and O1/O2 positions in accordance with the “10–20 system”.
🧠 🎯 10–20 SYSTEM – EXAM TIPS (AASM 3.0)
🧩 1. Start with the correct landmarks
📏 2. Golden rule: 10% – 20% – 20% – 20% – 20% – 10%
Must always equal 100%
Ends = 10%
Middle segments = 20%
💡 Exam tip:
Quickly check if proportions match this pattern
🧠 3. Electrode order (anterior → posterior)
👉 Fp → F → C → P → O
⚠️ EXAM TRAP: They may mix the order
🔢 4. Odd vs Even numbers (VERY COMMON)
Odd numbers (1,3,5,7) → Left side
Even numbers (2,4,6,8) → Right side
Z = midline
💡 Examples:
C3 = left
O2 = right
Cz = midline
📐 5. Cz location (classic question)
Cz is located at:
50% Nasion–Inion
50% between preauricular points
⚠️ TRAP: If it's described as anterior or posterior → ❌ wrong
🔍 6. Bilateral symmetry
F3 ↔ F4
C3 ↔ C4
O1 ↔ O2
💡 If not symmetrical → incorrect placement
⚡ 7. Common AASM montages (EXAM GOLD)
Know these by heart:
F4-M1
C4-M1
O2-M1
⚠️ TRAP: They may switch M1/M2 or sides
🧪 8. M1 / M2 references
M1 = Left mastoid
M2 = Right mastoid
💡 Example:
C4-M1 → right hemisphere referenced to left mastoid
🚨 9. Common exam traps
Incorrect 10–20 spacing
Misplaced Cz
Left/right reversed
Lack of symmetry
Wrong montage reference
🎯 10. PRO EXAM STRATEGY
When you see an image:
Find Cz first
Check symmetry
Confirm left vs right (odd/even)
Verify spacing (10–20 rule)
👉 This helps eliminate wrong answers FAST
🧠 MEMORY LINE
“10–20 symmetry, Cz in the center, odds left, evens right.”
