This paper gives a definition of, and summarizes orthographical issues related to, incidental letters/numbers/extensions. These are letters, numbers, and extensions appended to technical terms and forming an integral part of those. The given concepts, therefore, are not independently signalled without them, hence the name 'incidental'. Their coherence with the basic terms has to be indicated in writing, too, thus they are spelt solid (or hyphenated) with the basic term or acronym, etc. They significantly differ from attributive letters, numbers, and extensions, respectively; the latter are not part of the concepts referred to but merely convey additional information on some of their properties.
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