LIBRARYEEG Electrodes & the 10-20 System.
Where EEG electrodes go and why: the international 10-20 system, plus wet vs dry electrodes and the all-important reference and bias contacts.
Where you put an EEG electrode decides what you can measure, so the field standardized it. The international 10-20 system places electrodes at intervals of 10% and 20% of the measured distances between skull landmarks (nasion to inion, and ear to ear), so the same site means the same brain region across people and labs (Klem et al., 1999). Each position has a letter for the region it sits over (Frontal, Temporal, Central, over the central sulcus rather than a lobe, Parietal, Occipital) and a number, odd on the left, even on the right, with z marking the midline (e.g. Cz).
The 'C' (central) row runs over the sensorimotor cortex. C3 (left) and C4 (right) sit over the left and right hand motor areas, which is exactly why a motor-imagery BCI watches those two sites for the contralateral mu-rhythm changes. Cz, on the midline, sits over the foot/leg area. Standard names, standard physiology.
Wet, dry, and active electrodes
| Electrode type | Pros | Cons |
|---|---|---|
| Wet / gel (passive) | Lowest, most stable impedance; the clinical standard | Messy, slow setup, gel dries out |
| Dry | Fast, no gel, good for wearables/BCIs | Higher impedance; more motion artifact |
| Active (buffered) | Amplify/buffer at the electrode → tolerant of higher impedance and cable noise | More complex, needs power at the electrode |
The reference and bias electrodes
Two electrodes do a job people forget: the reference and the bias/ground. EEG is differential, so every 'channel' is really a measurement electrode compared against a reference (often an earlobe or mastoid, or a shared site); change the reference and the same brain activity looks different. The separate bias electrode carries the driven-bias / signal that cancels mains hum. Neither is optional: without a solid reference and bias, even a perfect amplifier gives you noise.
The biggest quality lever isn't the amplifier; it's the electrode-skin contact. Lower impedance with skin prep, and keep it BALANCED across electrodes (mismatch is what defeats common-mode rejection). A stable, well-matched contact beats a momentarily-lower but fluctuating one; a single hair or air gap under a contact can wreck a channel.
Checkpoint
Quick check
References
Keep going
Putting electrodes to work means a front-end that can read them cleanly: the build in the OTD Academy EEG front-end project.
One Thousand Drones Academy · reviewed June 2026
Coming soon
8-Channel EEG Front-End on ESP32 →Design the analog board that reads real brainwaves: the BCI.
Want the whole EEG & BCI cluster as one book?