What is the difference between 1020 and ABC electrode / cap labels?

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Electrodes and head caps can have positions derived from systems based on the International 1020 System.  When the positions are based on 1020, the labels on the caps and electrodes can either be the actual 1020 position names (such as Fp1, Cz, etc) or they can be what is called ABC, which actually means A1, A2, …A32, B1, B2,…B32, etc  Another way to refer to the ABC labeling is arbitrary alphanumeric.  The 1020 system is not arbitrary because the names are based on anatomical landmarks and the odd=left and even=right numbering is predictable and interpretable.  When you have only one type of cap in your lab, it makes sense to use 1020 labels for caps with positions based on the 1020 system.  However, if you have 32 channel caps AND 64 channel caps, then you might find it useful to be able to use the A1-32 labels for the A cable and the B1-32 labels for the B cable and complementary A1-32 labels on the 32 channel caps and A1-B32 labels on the 64 channel caps.  This way, the A cable can be used for the whole head in 32 channel studies of for just the left side of the head in 64 channel studies.  This does provide a short term savings if you don’t want to have two A cables on-hand, but using 1020 labels on everything is more convenient and at the limit it will not cost any more (because cables that are used less frequently will last longer).