Percentages, not centimetres
The 10-20 system does not place electrodes at fixed distances. It places them at fractions of two measurements taken on the patient: nasion to inion, front to back, and one preauricular point to the other, ear to ear.
The first electrode sits at 10 % of the measurement, the following ones every 20 %. Hence the name. On a 58 cm adult skull and a 42 cm child skull, the positions land on the same anatomical spot, while the distances in centimetres have nothing in common.
That is what allows a trace to be compared between patients, between hospitals, and on the same patient six months apart. Without that rule, Fp1 would mean nothing.
Reading a position name
The letter gives the lobe: Fp for frontopolar, F for frontal, C for central, P for parietal, O for occipital, T for temporal. A marks the auricular references, on the earlobe.
The number gives the side and the distance from the midline. Odd on the left, even on the right, and the larger the number the further from the middle. The letter z, for zero, replaces the number on the midline: Fz, Cz, Pz.
A classic montage has 19 active electrodes plus the two auricular references and the ground, so 21 or 22 channels. That is what the 21-electrode caps in the catalogue cover.
The reduced sleep montage
Polysomnography does not need the nineteen positions. The AASM rules ask for three derivations, frontal, central and occipital, referenced to the contralateral ear: F4-M1, C4-M1, O2-M1, with F3-M2, C3-M2, O1-M2 as backup.
Fitting a full cap for one night makes the technician and the patient work for channels nobody will look at. That is the reason sleep caps with ten or eleven electrodes exist, and the Sleepcap in particular.
To those EEG channels are added the EOG, the submental EMG and the tibialis EMG, which do not go through the cap but through adhesive electrodes.