Electrode derivations refer to the connection of two recording electrodes to a single amplifier differential input. In neurophysiology, generally neither one is "grounded", or at ground potential. If grounding the subject or prep is required, a third electrode should be used. It is generally essential to utilize the differential action of the "balanced" differential amplifier to minimize the interference due to 60 Hz power line frequency, especially at the high amplification required to amplify "low level" signals, such as EEG. An interfering signal that is "common" to both inputs, referred to as G1 and G2, will be rejected. The ability of the amplifier to reject the "common signal" is termed the amplifier Common Mode Rejection Ratio", or CMRR, and generally expressed as a ratio such as 10,000:1 or in decibels (dB) such as 80 dB, etc.

In EEG, multiple channels are required to obtain complete information about all of the areas of the brain (Frontal, Temporal, Occipital, Parietal). The pattern used to connect the recording electrodes to the amplifiers is referred to as a "montage". The montage is an electrode array. "Derivations" make up the montage. Several montages are generally used in "conventional" analog EEG while in "digital" EEG, a single referential montage is typically used, and various other montages are created digitally in the system.

Prior to 1947, anatomical placement of electrodes was left up to the individual lab. As more and more EEG labs opened, and comparisons of EEG data from lab to lab became desirable, it soon became apparent that data comparison was impossible due to inconsistent electrode placement. To solve this problem, the 10-20 or International Electrode Placement system was developed. The 10-20 system takes into account varying head sizes by measuring both the lateral distance between the ears and the longitudinal distance from the nasion (bridge of the nose) to inion (back of the head). These distances in millimeters are used to locate the standard 23 electrode positions by taking 10% and 20% values (hence the 10-20 system). This system is used all over the world. Note that sleep laboratories, although typically only recording 1-4 channels of EEG for sleep staging, use the 10-20 electrode placement for consistency.

Electrode positions in the 10-20 system are assigned alphanumeric symbols with letters identifying the location and number (odd or even) identifying the hemisphere (left or right). For example, electrdoe O1 identifies the left occipital, C3 the left central, P4, the right parietal, A1, the left ear reference, etc. Researchers working on some animals are not generally interested in the 10-20 system, and if requiring the ability to switch electrodes during the recording usually prefer the Numeric placement system, which simply provides a system using numbers of 1-21, A1, and A2 or if the expanded system. 1-21, A1, A2 and 26-37. We offer both types of systems.

See Grass educational aid A REVIEW OF THE INTERNATIONAL TEN-TWENTY SYSTEM OF ELECTRODE PLACEMENT.